Mazda’s new SKYACTIV gasoline and diesel engines are steps on the road to its “Ideal Engine”; focus on compression ratio
04 August 2011
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| Mazda’s roadmap to the ideal internal combustion engine. Click to enlarge. |
Mazda Motor Corporation’s SKYACTIV next-generation technologies—including engines, transmissions, vehicle bodies and chassis, launched in 2010 (earlier post)—represent the first key building blocks for the company in achieving its “Sustainable Zoom-Zoom” strategy, which initially calls for a 30% increase in fuel efficiency (compared to 2008 levels) for all Mazda vehicles offered worldwide by 2015. (This corresponds to a 23% reduction in fuel consumption and, therefore, CO2 output.)
From a powertrain frame of reference, Mazda is planning a building block strategy—i.e., the step-by-step introduction of electrification to SKYACTIV internal combustion engines: stop-start, followed by regenerative braking, followed by electric motor drive technologies. However, while Mazda plans to offer hybrid vehicles in the medium-term, its engineers are pushing hard to increase the efficiencies of both the gasoline and diesel platforms, moving toward what they call “the ideal Internal Combustion Engine.”
The initial SKYACTIV-G (gasoline) and SKYACTIV-D (diesel) engines that are being applied into new vehicles this year and next show Mazda’s approach to the goal.
Mazda hosted Green Car Congress at a SKYACTIV Technologies media workshop this week in Vancouver, Canada, at which engineering management explained the development philosophy, approach and results for the initial set of SKYACTIV technologies. In addition, the company provided a set of SKYACTIV mules (gasoline and diesel, manual and automatic transmissions) as well as current production vehicles to showcase on the road the results of its development teams’s efforts.
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| Energy balance in an internal combustion engine. Click to enlarge. |
Engineering the ideal internal combustion engine. Even after 120 years of non-stop development, the internal combustion engine still fails to utilize even close to the majority of the energy contained in the fuel. Since this energy loss is primarily thermal in nature and can be attributed to the exhaust, cooling system, and engine and transmission surfaces, Mazda’s R&D team’s central focus is on improving the engine’s thermal efficiency cost-effectively. Beyond that, Mazda has also been busy working to reduce internal engine friction as well as engine weight.
At the workshop, Kiyoshi Fujiwara, Mazda Executive Officer in charge of project planning and development, suggested unofficially that Mazda engineers think they ultimately may be able to approach 60% thermal efficiency with combustion engines for light duty vehicles.
| SKYACTIV at Powertrains, Fuels & Lubes |
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Mazda engineers are presenting a series of papers on SKYACTIV technologies at the upcoming 2011 JSAE/SAE International Powertrains, Fuels & Lubricants meeting in Kyoto at the end of August. These include:
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The six controllable factors at the heart of Mazda’s approach are:
- compression ratio
- air-to-fuel ratio
- combustion duration
- combustion timing
- pumping loss
- mechanical friction loss
Mazda’s goal is to optimize these factors. Ultimately, the compression ratio ended up playing a central role among these factors in both gasoline and diesel engines, although in different directions.
Extreme compression ratio rather than downsizing. Although downsizing—i.e., reducing displacement to improve fuel economy and offsetting the resulting loss of power and torque via charging—is an effective approach to reducing fuel consumption, Mazda has chosen a different route.
According to Mazda’s roadmap for the ideal engine, the most effective next step was to optimize the compression ratio. For the SKYACTIV-G engine, this meant increasing the compression ratio to 14:1 (13:1 for North America). For the SKYACTIV-D, this mean reducing the compression ratio to 14:1.
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| SKYACTIV-G fuel consumption. Click to enlarge. | SKYACTIV-G torque. Click to enlarge. | |
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| SKYACTIV-D fuel consumption. Click to enlarge. | SKYACTIV-D torque. Click to enlarge. |
SKYACTIV-G
The new 2.0L SKYACTIV-G engine will deliver approximately 15% lower fuel consumption and CO2 emissions than the current Mazda 2.0-liter MZR gasoline engine, with approximately 15% more torque at the lower and mid-ranges, using 87 AKI (anti-knock index, or pump octane) fuel.
Raising the compression ratio in a gasoline engine increases its thermal efficiency, thus improving fuel economy. However, high compression in conventional engines leads to unwanted abnormal combustion (i.e., knock) and an associated reduction in torque. A richer mixture and delayed ignition timing are used to avoid knocking, but these also come at the expense of fuel economy and torque.
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| Residual gas reduction via the 4-2-1 manifold system. Click to enlarge. |
Knocking takes place when the air-fuel mixture ignites prematurely because the temperature and pressure are too high. This can be countered by reducing the quantity and pressure of hot residual gases in the combustion chamber. Mazda, in response, developed a special 4-2-1 exhaust manifold, which, due to its relatively long structure, prevents the exhaust gas that has just moved out of the cylinder from being forced back into the combustion chamber. The resulting reduction in compression temperature inhibits knocking.
The combustion duration was also reduced. Faster combustion shortens the time the unburned air-fuel mixture is exposed to high temperatures, which enables normal combustion to conclude before knocking occurs.
| Rollout |
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| Mazda said it would bring a Mazda3 equipped with SKYACTIV-G engine and SKYACTIV transmissions to North America later this year. |
| The full range of SKYACTIV technologies (i.e., including body, chassis, suspension) will then first appear in the new CX-5 in 2012. |
| Mazda said it is weighing bringing the diesel to North America as well. |
The new engine also received special piston cavities, which allow the initial combustion flames to propagate without interference, and new multi-hole injectors, which enhance fuel spray characteristics. Together with the 4-2-1 exhaust manifold, these innovations resulted in a substantial 15% increase in torque over Mazda’s current 2.0-liter MZR gasoline engine.
The SKYACTIV-G also features a smaller bore: 83.5 mm compared to 87.5 mm in the current 2.0L engine. The smaller bore reduces cooling loss and contributes to improved thermal efficiency.
Mazda managed to minimize pumping loss (20% reduction) with a continuously variable dual S-VT (sequential valve timing) system on the intake and exhaust valves, enabling the air intake quantity to be controlled by the valves rather than the throttle. During the intake stroke, the throttle and intake valves are kept wide open while the cylinder moves downward. The intake stroke finishes when the piston reaches bottom dead center(BDC). But if the intake valves close here, there is too much air inside the cylinder when only a small amount of air is needed at lower engine loads. In order to push out the excess air, the intake S-VT keeps the intake valves open when the piston starts to move upward during the compression stroke. The intake valves then close when all unnecessary air is pushed out.
The S-VT system is supported by the adoption of a compact electronic variable-pressure oil pump.
A drawback to this process is destabilized combustion. Since the intake valves are kept open even when the compression stroke starts, the pressure inside the cylinder decreases, making it difficult for the air-fuel mixture to combust. However, the high compression ratio in the SKYACTIV-G increases combustion chamber temperature and pressure, so the combustion process remains stable—despite reduced pumping loss—and the engine is more fuel efficient.
The SKYACTIV-G engine also features 20% lighter pistons, 15% lighter connecting rods and a 30% reduction to internal engine friction compared to the current 2.0-liter MZR engine.
SKYACTIV-D
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| Higher expansion ratio due to lower compression ratio. Click to enlarge. |
The 2.2L SKYACTIV-D engine reduces fuel consumption compared to the current 2.2L MZR-CD diesel by 20% due to a low 14:1 compression ratio and subsequently greater expansion phase after combustion. SKYACTIV-D is also one of the first diesels to comply with Tier II Bin 5 North American emission regulations without requiring expensive selective catalytic reduction (SCR) aftertreatments or a lean NOx trap catalytic converter (LNT).
Reducing the compression ratio in the diesel decreases compression temperature and pressure at TDC. Consequently, ignition takes longer even when fuel is injected near TDC, enabling a better mixture of air and fuel. The formation of NOx and soot is alleviated since combustion becomes more uniform without localized high-temperature areas and oxygen insufficiencies, Mazda says. Furthermore, injection and combustion close to TDC make a diesel engine highly efficient. The expansion ratio (or amount of actual work done) is greater than in a high-compression diesel engine.
| Mazda and hybrids |
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| Toyota Motor Corporation and Mazda Motor Corporation reached an agreement in 2010 on the supply of the hybrid technology components, upon which the Toyota Prius is based. |
| Mazda plans to combine this hybrid system with its next-generation SKYACTIV technologies to develop and introduce a hybrid vehicle in Japan, starting in 2013. |
| The fuel efficiency of today’s engines decreases significantly from medium to low loads at low engine speeds. Hybrids can deliver good fuel economy by powering the vehicle at lower loads. However, says Mazda, the wider the internal combustion engine’s inefficient lower load range is, the larger a hybrid’s electric motor and battery need to be to compensate for it. |
| Mazda intends to leverage the efficiencies of the SKYACTIV internal combustion engines to enhance overall hybrid effectiveness with a lighter electric motor and battery. Regenerative braking can thus serve as the predominant source of power to charge the battery. |
| Mazda intends to leverage the efficiencies of the SKYACTIV internal combustion engines to enhance overall hybrid effectiveness with a lighter electric motor and battery. Regenerative braking can thus serve as the predominant source of power to charge the battery. |
Also due to the low compression ratio, the SKYACTIV-D diesel engine also burns cleaner, discharging far fewer nitrous oxides while producing virtually no soot. It can thus do without NOx aftertreatments and still meet emissions standards globally.
However, there are drawbacks to a low compression ratio in a diesel engine. The compression-ignition temperature for cold starts and during cold operation is normally too low in a diesel engine with a compression ratio of only 14:1. The diesel would run rough, particularly in winter conditions, misfiring during the warm-up phase, and at extremely low temperatures, the engine might not start at all.
To improve cold starting and cold running, SKYACTIV-D diesel engines are furnished with ceramic glow plugs as well as exhaust variable valve lifts (VVL). The role of the latter is to allow the internal recirculation of hot exhaust gas into the combustion chamber.
A glow plug is used to carry out the first combustion cycle, which is enough to raise the exhaust gas to a sufficient temperature. After the engine starts, the exhaust valve does not close as usual during the intake stroke. Instead, it remains slightly open to allow some exhaust gas to re-enter. This increases the air temperature in the combustion chamber, which in turn facilitates the subsequent ignition of the air-fuel mixture and prevents misfiring.
SKYACTIV-D’s lower compression ratio also means lower maximum pressure and less strain on engine components than in conventional diesels. Maximum combustion pressure is 20% less than the current 2.2L diesel (130 kg/cm2) compared to 170 kg/cm2, or 1860 psi vs. 2430 psi).
| Mazda EV |
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| An electric version of the Mazda2 will be offered in very limited numbers in 2012 in Japan as part of a leasing program. |
This in turn allows room for structural modifications to further reduce weight: cylinder heads with thinner walls and an integrated exhaust manifold are 6.6 pounds (3 kg) lighter while the new aluminum-made cylinder block saves another 55.1 pounds (25 kilograms).
With another 25% decrease in the weight of the pistons and crankshafts, Mazda managed to reduce overall internal engine friction by 20% in the SKYACTIV-D diesel engine relative to the current MZR-CD diesel.
SKYACTIV-D also utilizes two-stage turbocharging. A small, quick-responding turbo feeds air to the combustion chambers at low engine speeds to provide low-speed torque and eliminate turbo lag.
Interesting how ICE mpg improves after gas-free, ~maintenance-free EV's start cruising down the road.
Posted by: kelly | 04 August 2011 at 03:18 PM
These technologies have been in development for many years now, and by numerous automakers. GM, for instance, was working on HCCI long before the Volt was even a concept. Ford's Ecoboost was also a step in this direction, and that effort was launched several years ago already.
There is growing demand for fuel economy today and increasing regulations. 5 years ago, on the other hand, consumers cared more about the placement of cupholders than fuel economy. That made fully embracing these technologies difficult because it was cheaper in the short term to embrace the status quo.
When you produce several million vehicles every year, why throw a wrench into the production system unless you have to? That's how automakers think.
Automakers could have made cars much more fuel efficient 20 years ago, but they didn't because fuel economy didn't drive the market. Today its becoming a bigger driver.
At the end of the day automakers are in business to make a profit, and these new technologies are looking more and more profitable.
Posted by: Chad Snyder | 04 August 2011 at 04:01 PM
The SkyActive-D strikes me as being particularly innovative: nothing about it is exotic or expensive. It's just a smart design. Hopefully they combine it with a lighter version of the Prius' hybrid system and put it in an even more aerodynamic version of the Mazda 3 for some 60+mpg action at a reasonable price.
We're headed down the other side of Hubert's peak. We need all hands on deck. It's nice to see that Mazda is being practical about this.
Posted by: GreenPlease | 04 August 2011 at 09:16 PM
Skyactive-D follows a trend that started in after Audi introduced the first successful DI diesel engine in late 1989. The compression ratio then was 21:1; now it has, on average, been reduced to ~16:1 and some engines are even lower. A level of 14:1 is certainly the lowest at the moment.
Probably the most interesting feature of the Skyactive-D engine is that it can meet US T2B5 and Euro 6 without NOx aftertreatment. For a couple of years, I have stated that this is (will be) technically possible and it is nice to see that it finally happens.
One issue I have difficult to understand is that the low cylinder pressure level is not utilized for downsizing. One particular problem when you increase specific torque and power is the accompanying increase in cylinder pressure. Of course, engine manufacturers try to increase the pressure capability of engines but it is becoming increasingly difficult. Current state-of-the-art level is 200 bar (Mercedes 2.15-liter engine; Audi & BMW: 185 bar) and this could be extrapolated to, perhaps, ~250 bar with steel pistons and several other features. Mazda claim only 130 bar (170 bar in the previous engine), which is outstanding even if we take into account that this engine has “only” 420 Nm in torque compared to 500 Nm for the class-leading Mercedes engine. Why not design the engine for high cylinder pressure and use this margin for further increase in torque/power and/or downsizing? Increasing the maximum cylinder pressure to a “moderate” level of 180 bar would enable reducing engine size to 1.6 liters while retaining power and torque. The weight increase due to stronger engine structure could easily be compensated if engine size is reduced. In a comment in a different thread (see below), I discuss the issue of high cylinder pressure.
http://www.greencarcongress.com/2011/06/coolr-20110623/comments/page/2/#comments
Posted by: Peter_XX | 05 August 2011 at 02:09 AM
Chad, "These technologies have been in development for many years now, and by numerous automakers." and would/will have been for many more years..
Posted by: kelly | 05 August 2011 at 06:53 AM
I am thoroughly impressed by Mazda's SKYACTIVE development programme. Particularly because this seems like real 'oldskool' engineering when it comes to engines. I love how they play with the cycle parameters, rather than just attach the newest and bestest turbo... (not to discredit other developers) It follows from their strategy that they can only afford cheap options and only 2-3 engine types.
I have noticed how improvements in exhaust systems have been instrumental in a lot of gasoline engine development. Ford Ecoboost and the recent Audi 1.8T credit improved exhaust cooling systems for reaching higher turbo boost pressure. But I find Mazda's 4-2-1 particularly ingenious because it allows a marked increase in compression ratio with a simple and elegant solution.
Peter_XX, you also say that you wish Mazda would increase the specific torque by 20% to Mercedes levels (in my interpretation) to allow downsizing. But the reduced cylinder pressure is excactly the source of their efficiency improvement, because colder pre-ignition temperature allows earlier fuel injection and combustion and a resulting increased expansion stroke, which I am sure you understood. But I do not see how it would be possible to combine those features. Furthermore, they utilize the reduced engine pressure to use cheaper and lighter materials, leading to a welcome 28 kg weight reduction - something that matters in a car of this size. (Obviously, downsizing could accomplish the same...)
They also mention significantly reduced NOx formation as a positive side effect of the slower but prolongued combustion phase.
But perhaps the reason is different altogether. Mazda may have learned that they can make a 2.0 litre diesel with the same (or better) fuel economy as the competitors' downsized 1.6 - and at the same cost and weight. However, costumers may prefer the '2.0' tag on the rear over a '1.6'. Anyway, it leads to brand differentiation and allows Mazda to tell a different story than everyone else...
My guess is that it is a combination of all of the above since engine optimization is truly a multi-dimentional discipline!
I would like to complement Mazda on their intelligent R&D and GCC for an excellent article!
Posted by: Thomas Pedersen | 05 August 2011 at 08:22 AM
@Peter
It seems that Mazda has taken the approach that smaller is not necessarily better. I guess, in their view, combustion efficiency and post engine pumping losses (read: exhaust after-treatment) take a priority to overall pumping and heat losses.
Also, displacement is cheap (add another or a bigger turbo).
Posted by: GreenPlease | 05 August 2011 at 09:16 AM
Thomas
I have spent most of my life in this business, so I think I have some basic understanding of engines. Let me simplify: If you increase the specific torque by 20%, you can also increase both pressure before combustion start and maximum cylinder pressure by 20%. Thus, the specific fuel consumption will be exactly the same in this simplified analysis. Combustion speed and center of gravity for the combustion curve will be similar (if you are not familiar with these expressions, just note that combustion rate, and timing will be “the same”), so fuel consumption is not affected. This is just simple mathematics but it can be supported by experimental evidence as well. More important, fuel consumption at light load will improve due to the downsizing effect. This is of greater importance in real-life driving than top efficiency (although this is not affected). Other manufacturers use this approach. One example is the new Renault 1.6-liter engine that replaces an older 1.9-liter engine with significantly reduced fuel consumption as result.
Engine weight? The main reduction in mass (25 kg) was due to the change in cylinder block material. The structural change of the cylinder head and integrated manifold reduced weight by a mere 3 kg, which cannot impress anybody. Mazda does not mention the total weight of the engine, but I would be surprised if it was lighter than, for example, the BMW 2-liter engine at 148 kg. This engine can handle 180 bar and I bet that they will hit 200 bar pretty soon. What I recognize in the Mazda case is the great reduction in cylinder pressure due to the low compression ratio.
Fuel consumption? If we want to make a fair comparison, we should not compare with a 1.6-liter engine that has lower power. Let’s take a 2-liter engine of similar power, for example a VW Passat or a BMW 320d for comparison. However, I have not yet seen any data on a Mazda car with this engine. The previous engine had very high fuel consumption, so it should be pretty easy to improve from that level.
GreenPlease
Yeah, it is nice to see that one a manufacturer can have different approach than the others. The ultimate test is of course if buyers will like the cars. Before we know that, it will be interesting to see how the fuel consumption for cars with this engine compares with the best competitors.
Posted by: Peter_XX | 05 August 2011 at 10:19 AM
Another way to increased efficiency is to use Atkinson-like cycle in diesel engine - this separates compression ratio from expansion one and allows to expand gases to the level just sufficient to drive the exhaust.
Posted by: yarross | 06 August 2011 at 12:44 PM
@yarross
Yes, Atkinson cycle or Miller system is a good option. This will also reduce the maximum cylinder pressure, which is an increasing problem (albeit not so prominent for the Mazda engine, as discussed above). However, Atkinson/Miller put more stress on the turbocharging system, i.e. higher pressure is needed but less exhaust energy is available. One option could be to move from the – basically sequential – layout of contemporary bi-turbo system to a “true” two-stage system, as used on heavy-duty engines.
Posted by: Peter_XX | 07 August 2011 at 01:21 AM
Peter_XX,
I get what you are saying, but to me it sounds like a direct contradiction to the claim of the Mazda engineers:
"Reducing the compression ratio in the diesel decreases compression temperature and pressure at TDC. Consequently, ignition takes longer even when fuel is injected near TDC, enabling a better mixture of air and fuel. The formation of NOx and soot is alleviated since combustion becomes more uniform without localized high-temperature areas and oxygen insufficiencies, Mazda says. Furthermore, injection and combustion close to TDC make a diesel engine highly efficient. The expansion ratio (or amount of actual work done) is greater than in a high-compression diesel engine."
I am curious, do you think the Mazda engineers are wrong. Or do you see a way to combine low initial temperature, slower ignition and prolonged mixing with higher mean pressure (e.g. with higher turbo boost and stronger intercooling)? Or is there a third option I do not see?
Maybe you are right that downsizing like everyone else is a better idea. And like you said, improving the efficiency of their diesel could not have been too hard when their current gasoline has lower BSFC than their previous diesel...
Anyway, while I completely understand the reasons for and benefits of downsizing (higher loading and less cooling loss, among others), it is interesting to see someone going in the opposite direction and hear their reasons for it.
Posted by: Thomas Pedersen | 08 August 2011 at 08:53 AM
@Thomas
Where is the contradiction? “Inject near TDC” can mean before or after TDC. Of course, ignition delay is longer with a lower compression ratio. Therefore, they can inject earlier. Thus, “earlier injection” would have been better wording from Mazda. If the crank angle for the center of gravity for combustion rate (the best “simple” indicator we have) would be the same for an engine with high compression ratio, the expansion ratio would also be higher for the engine with high compression ratio. This is as a law of physics. Presumably, Mazda can inject so early that they overcompensate for this drawback (i.e. the lower geometric compression ratio) and actually get somewhat better expansion ratio than in a conventional diesel engine. However, they provide no data to show that. To give yet another “free lesson”, I can tell you that injection often is “too late” (for several reasons) in contemporary diesel engines for best efficiency. The Mazda engine is closer to this optimum and this compensates for the low geometric compression ratio, as already described above.
Mazda engineers are not wrong. I did not say that. My point is that they could make an even better engine if they downsized as well. I also provided facts to prove my point. Do you deliberately want to misinterpret everything? I am also curios, do you think that the engineers at all other car manufacturers (who advocate downsizing) are wrong.
I am not impressed with the fuel consumption of the old Mazda diesel and gasoline cars. It is easy to improve from that level. However, I have not seen any comparison of BSFC between their current gasoline engine and their previous diesel engine. Have you? In addition, the car itself and other parts of the drivetrain but the engine has numerous improvements, so you cannot compare apples and pears by looking at fuel consumption between one old and one new car and claim that this is due to engine improvement alone. As I said, Mazda has not provided any data on BSFC on their diesel engine in the article. The diagrams are without scales. Why, one could ask? I would be very surprised if they could beat the sub 200 g/kWh in the best operating point that the best European engines achieve. European manufacturers often show diagrams with scales! Furthermore, there is no gasoline engine on the market that can come even close to 200 g/kWh. Energy content of gasoline and diesel fuel per kg is so close that we can compare the numbers directly without any correction (e.g. gasoline BSFC as diesel-equivalent BSFC in g/kWh) and not make too much error. Look at an engine map for a gasoline engine and you often see much higher numbers than 200 g/kWh in the best point. Therefore, I would like to see some hard facts (i.e. graphs with scales) on the Mazda engines.
Posted by: Peter_XX | 08 August 2011 at 01:45 PM
Peter_XX,
In the paragraph that I cited, they say that low compression ratio leads to (comparatively) low pre-ignition temperature, leading to delayed ignition. This is a central part of their 'innovation' if you will call it that. I am well aware of all the other factors that would be similar with higher compression ratio and mean pressure, but this specific point (low pre-ignition temperature) cannot be the same with high compression ratio. And with faster ignition in a downsized engine with higher compression ratio, peak temperatures would soar leading to greatly increased NOx formation. I assume this is the reason for the "too late" injection in contemporary efficient diesel engines.
In no way have I said that all other diesel manufacturers are wrong. Do you deliberately misinterpret everything? I said that it is interesting to see someone take a different approach.
It would be even more interesting, if they were brave enough to publish actual BSFC figures - that I agree with. Their powerpoint-drawn figure shows the gasoline levels being lower than the previous diesel engine (top-left figure). I also agree that this is probably more testament to their less-than-impressive fuel economy in the previous version, but still nice work with their gasoline engine.
The fact that this engine is Tier II Bin 5 compliant without NOx after treatment indicates that this was a crucial design parameter for Mazda, now that I think of it. If Mazda can avoid NOx after treatment with lower compression ratio and compensate for lower efficiency by improved expansion, then that might place them more favourably in the market with additional fuel costs being more than offset by reduced power train costs.
Furthermore, as shown in the newer SKYACTIVE article, Mazda set out with a target of 100 kg weight reduction of the whole vehicle (including the 28 kg savings from the engine, presumably). That along with improved transmission efficiency (automatic), I suspect, was a cheaper solution for Mazda to improve fuel economy than engine downsizing. Obviously, the two are not mutually exclusive. But with a constrained dollar budget - and a top level commitment to vehicle weight reduction, I would not be surprised if that played into the strategy for their engine development. But that is all speculation, obviously.
Posted by: Thomas Pedersen | 09 August 2011 at 01:57 AM
@Thomas
O.K. this time I agree with most of your comments. One issue you might have got somewhat wrong, though. Downsizing does not necessarily have to result in higher in-cylinder temperature, provided that the charge cooler is good enough. If inlet temperature is the same, the temperature before combustion will be essentially the same, regardless of engine size. Actually, a smaller engine has slightly higher heat loss that marginally reduces the temperature. However, as said before, pressure increases, which is what we want. This increase has a small impact on ignition delay but it will still be longer than for any other contemporary engine. Temperature dominates ignition delay. Still afraid that the ignition delay would be reduced? Well, the simple answer is to do a marginal reduction of the compression ratio to get exactly the same ignition delay. I already demonstrated that this engine could be downsized significantly without exceeding limits for cylinder pressure in contemporary engines. Are there any further problems with downsizing? No, not any direct! Let me add, though, that current turbocharging will be a limitation for too aggressive downsizing. Therefore, it may not yet be possible to go down to 1.6 liter but the range of 1.8 to 2.0 liter seems feasible. I am sure that Mazda engineers would be clever enough to copy their competitors (e.g. Mercedes) when it comes to turbocharging, if necessary. BTW, the Mazda engine already has bi-turbo. With the same specific torque as the Mercedes engine, 1.8 liter would be feasible for similar performance as the 2.2-liter engine. Still, the maximum cylinder pressure (~160 bar) would be lower than for the Mercedes engine (200 bar) and some other competitors (185 bar).
So, why did not Mazda downsize the Skyactive-D, when they had such a good opportunity? I can think of two major reasons and none of them are technical: 1) They already have the production facilities to produce the 2.2-liter engine and did not want to invest in a new one. 2) They might have reserved some margin for a version with higher power to be introduced later. At, let’s say, about 230-250 hp, this engine would be a contender to competitor’s 6-cylinder engines.
Posted by: Peter_XX | 09 August 2011 at 04:11 AM
Peter_XX,
Downsizing is most likely better than the Mazda concept regarding power output and fuel efficiency. I think we actually agree on that at this point. I also seems likely to me that 'lowest-in-the-market' fuel consumption was never the objective for Mazda, who has freely admitted that they cannot keep up with big companies regarding engine development. I do not think they want to get into a BSFC contest with the German premium brands. Furthermore, there is no way Mazda can demand the same premium for an exhaust after treatment system necessary for downsized engines as Audi/BMW/Mercedes, whose costumers revel in owning the best technology.
What Mazda has achieved is a cost effective Tier II bin 5 diesel engine for the US and global markets. And with room for a stronger version, as you say.
I thought about retooling cost, but it seems to me that reducing displacement from 2.2 to 2.0 and changing block alloy is enough changes to facilitate complete retooling - although I'm not expert. But steel and aluminum have different vibration characteristics, so reinforcement ribs and other anti-resonance features on the engine block would have to be redesigned anyway.
PS. In my comment about downsizing I was assuming similar torque in the large and small engine. All things being equal, as in our discussion, this means higher mean pressure in the smaller engine, facilitated by higher compression ratio. Assuming that Mazda already does their best in the intercooler and that temperature before compression is the same in both cases, temperature at TDC will be higher. If, on the other hand the pressure is raised by keeping compression ratio constant and increase the turbo boost pressure, yes, the temperature would be the same - as I was inferring in our Audi 1.8 discussion.
Posted by: Thomas Pedersen | 09 August 2011 at 06:11 AM
@Thomas
About temperature: Now, I really tried my best to explain that you can have the same temperature also with a downsized engine. Apparently, you did not understand. However, now I am so sick and tired of this discussion that I will not try again… This will be my final comment on this topic. Let me suggest that you make your own experiments. You might experience a couple of surprises if you test your hypothesis.
Posted by: Peter_XX | 09 August 2011 at 12:01 PM
Peter_XX,
Apologies, it seems I got into my head that you meant to increase cylinder pressure with higher compression ratio, when upon a closer re-read of your posts, you mention nothing of that. That was the source of our disagreement, it seems...
I completely agree that with better turbocharging is is possible to increase the pressure without increasing the temperature. This was exactly my point in our other discussion about the Audi 1.8 engine.
I also see - much to my dismay - that I said it is not possible to increase the pressure without the temperature. Again, I was actually referring to pressure ratio. I apologize for the inaccurate writing and the time we wasted discussing basic thermodynamics - which we both understand very well it seems.
And as you say, judging from the low cylinder pressure in this engine, there should be room for higher boost pressure. Especially with a twin turbo solution!
Posted by: Thomas Pedersen | 09 August 2011 at 02:21 PM
There are many more ways to improve past and current low efficiency ICE.
Many would agree that car-truck manufacturers have not tried hard enough in the last 60+ years as the went from I-4 cyls to V10 and V-12.
Many would agree with Kelly that the arrival of HEVs, PHEVs and BEVs may have something to do with the recent success with ICEs higher efficiency.
Posted by: HarveyD | 10 August 2011 at 06:05 PM