"Peak output of 125 kW is achieved at 3,800 rpm"

That is one low-revving petrol engine, lower even than most diesels!

And diesel-like torque, too.

I knew that direct injection have not just advantages so they use both direct and indirect, that's a good engineering piece. The germans seams efficient these days with their audi windmills e-gas project, this engine and also the bmw i3, will they finally win the third world war( gasoline price at the pump) , they are on their way to eradicate the big oil cartel own and operated internationnally by swiss bankers with their secret black market private banks accounts. 2 world war for nothing, 1914-18 and 1939-45 for nothing because gasoline price is prohibitive and hack by this swiss corrupted internationnal petrol cartel. We don't need petrol except in old and actual cars. We need hydrogen cars, trucks, ships, electrical stations and airplanes and germany seam to tackle this tech better then usa,japan because these countrys financial elites are still casching petrol money in secret swiss bank accounts.

PeterXX,

Where in the engine cycle is indirect injection superior to direct in-cylinder injection?

ExDemo
Where in the engine cycle? I am not sure I understand your question but I will give some background… The engine has 4 strokes, in two engine revs, i.e. 720°. Injection timing is completely different for both types of injection systems and has to be so. With indirect injection, much longer time is available for air/fuel preparation if injection is early in relation to the combustion stroke. This gives more homogenous mixture, which is advantageous under some operating conditions (e.g. low engine load). On the other hand, injection during the induction stroke alone is problematic regarding homogenization. Likewise, fuel deposition in the inlet system is not easy to control during transients. Direct injection makes use of the so-called “charge cooling” effect and can utilize higher compression ratio without knock, with lower fuel consumption as a benefit. Charge stratification can also be used to get a lean mixture – or more exhaust gas recirculation – for reducing fuel consumption but the former alternative has not been used by Audi, probably due to NOx control problems. With DI, the “window” for injection (in crank angle degrees) is much smaller and must be timed with regard also to both piston position and air movement, which cannot all the time be optimized. Thus, indirect injection can be better under some operating conditions. However, if direct injection could be made “perfect” under all operating conditions, there would be no need for two injection systems. Some researchers have started to investigate high injection pressures, i.e. in the 500 to 1000 bar range (compared to current state-of-the-art level of 200 bar).

Besides the theoretical aspects listed above, the statements by Audi in the papers I mentioned are important, although they do not discuss the topic thoroughly. Indirect injection is used by Audi at low engine loads; direct injection is used at high load and during engine starts. One advantage mentioned is lower particle emissions (mass and number) with indirect injection. Recall the more homogenous air/fuel mixture with indirect injection, as I mentioned above, which reduce soot formation. Thus, Audi states that the engine will be able to fulfill the (proposed) Euro 6 limit for particle number emissions. It now appears that a much higher level for number of particles will be allowed for gasoline cars than for diesel cars and meeting this limit without a gasoline particle filter (GPF) could justify two injection systems. If only direct injection would be used, a GPF might have to be fitted with a significant cost increase as the result.

If you want more detailed information about the benefits of double injection systems, I recall that Toyota has provided much more on this topic in a paper on one engine (V8) that also use this feature. I have to look for that paper myself, since it was a long time ago when I read it…

PeterXX,

You misunderstood, or did not read what I wrote. I said that is has more (specific) torque than *their* 2.0 L diesel. I am well aware that VW/Audi are behind the curve regarding 2.0 diesels, even though their 2.0 has been upgraded to 177 hp in the new Audi A6.

Diesels still allow higher torque than gasoline engines because of higher compression ratio, and slower combustion, which allows the maximum cylinder pressure to be maintained for a longer period. However, compression ratio is just one way to ensure high cylinder pressure (torque). A decent charge air cooler will deliver the incoming air at the same temperature regardless of the charge air pressure. So with a good turbo (or supercharger), the cylinder pressure before ignition can be the same in a gasoline engine as in a diesel. This effect is exactly what we see in this engine, made possible by new turbine alloys and more expensive construction of the compressor wheel.

I would like to add to Peter's comment about gasoline injection pressure that while state-of-the-art diesel injection pressure is now around 2000 bar, gasoline has extremely poor lubrication properties (much worse than water), making it difficult to increase pressure in a pump that is both economical and durable.

I would also like to add that I fail to see how direct injection accoplishes more charge cooling than indirect injection..? Unless it is about local temperature rather than cylinder average temperature.

Peter XX,

Thanks for your explanation.

As I thought, only with very cheap first generation DI is there any possibility of indirect injection being superior for portions of the fueling cycle, and engine operation.

A modern high pressure direct injection setup with multiple and controlled injection events in a cycle such as FIAT uses in its "Multi" Air or "Multi" Jet approaches for Otto and Diesel ICEs respectively, eliminates the necessity for a dual, and redundant second complete set of indirect injection equipment.

IOW, this Audi engine is a kludge; and a hodge podge of old ideas, producing little comparable HP, with a very limited rev range, while costing an excessive amount of money and componentry to construct.

It might sound that I express a negative position but the Fiat multi-air, BMW valvetronic or any other gimmick today cannot “fix” the shortcomings of current gasoline direct injection. That is, if we want good enough atomization and mixing at molecular level to achieve the low solid particle number (SPN) emission level that we get with particle filters (DPF) on diesel engines (but without a filter in the gasoline case). If we want to achieve that SPN level, the contemporary solution would be to revert to indirect injection, with higher fuel consumption as a result (which nobody wants). However, considerable progress has been demonstrated by optimization of the injection event (multiple injections, timing, etc.), such as in a recent publication by the consultant company AVL. It is also up to the EU to set the SPN limits. If they would set a similar limit for SPN as for diesel cars, it is simply not possible to meet that level with DI. If the limit is set at a higher level (e.g. up to 10x diesel), optimization of the DI might be sufficient to meet that limit. Dual injection system, such as in the Audi case, will provide much more freedom in optimization and will achieve an even lower level (as indicated by Audi) but still not as good as a diesel with DPF. As current information suggest, EU will set a higher limit for gasoline cars than for diesel cars but, in this case, I do not have any inside information about the exact level to be proposed. However, on the long term, it cannot be “fair” to allow much higher SPN emission level from gasoline cars than from diesel cars, so I presume that the level will become more stringent in later regulations (e.g. Euro 7, 8…). This will be a driving force for further development of direct injection systems; it will not stop at 200 bar, which engineers refer to as second generation (1st @120 bar). I have hard to believe that double injection systems will be the preferred solution in the long run. I do not think manufacturers will revert to indirect injection alone either, so there is a good opportunity for fuel injection suppliers to continue development in this field. The gasoline particle filter (GPF) is not as efficient as the DPF counterpart (I will leave the explanation out this time) and will add to the cost considerably. However, I would not completely rule out this solution either. With very tough SPN limits (e.g. Euro 7…) and limits also for “off-cycle” driving conditions, a GPF might be necessary. It might also be possible to integrate the GPF function in the catalyst. If this could be accomplished at low incremental cost, GPF might even be an attractive solution in contrast to expensive injection solutions.

Sure, there are other options...

Peter_XX,

My mistake - the latest Audi A6 2.0 diesel has a specific torque of 190 N·m/litre, versus 177 for this gasoline. I was referring to the Audi/VW 2.0 diesels with either 320 or 350 Nm, both lower than this new gas engine and still the bread and butter of their sales.

Regarding initial pressure before ignition in gasoline versus diesel: While I find your tone offensive, I would still like you to comment on while it is not possible. I am not an automotive combustion engineer, so I do not have access to detailed autoignition curves (usually hidden behind '$20 firewall'). With the limited time I have had to research the issue, I have not been able to fine any pressure dependence, nor understand where it comes from. Autoignition is obviously strongly dependent on chemical composition and temperature, but I have not been able to see where the pressure dependence comes from. Perhaps you could educate us?

I was stating that turbo/supercharging, followed by intercooling and then compression with low compression ratio can achieve the same pressure before ignition as in a diesel, without exceeding autoignition temperature.

Whether that would be feasible is a whole other matter... But as a possible way for gas engines to catch up to diesels, I find it interesting, although it relies on expensive turbos, as in the case of this Audi 1.8 engine.

PS, thanks for the explanation about charge cooling of indirect injection being eaten by hot surfaces. That makes sense.

PPS. Since gas engines have higher Max/ave pressure than diesels during the expansion stroke (isochoric vs isobaric combustion), might this engine have higher maximum pressure than contemporary diesels?

Peter_XX

It seems we are slowly converging to understanding each other...

The 177 hp Audi has 380 Nm (190 per litre). I was not aware there was a 400 Nm (200 per litre?) version as well. Which model?

I never meant to imply that contemporaty gasoline and diesel engines have the same pressure before combustion. I know better than that. My point was that it is theoretically possible (by higher turbo boost in the gasoline engines than in diesel engines), although gas engines have a long way to go.

In the case of this Audi engine, the absolute pressure would be 2.3 bar (multiplied by inlet suction factor) for the gasoline engine and I have no idea what a small diesel engine can achieve. I know the large ship engines can be equipped with turbos capably of up to 5 in pressure ratio (single stage with titatium compressor wheel), but I suspect car turbos are much less efficient. Since diesel exhaust is colder there is less potential for work. On the other hand, diesel exhaust pressure might be higher, and a colder turbine can run faster with better aerodynamic efficiency under real world conditions. You indicated 2.6 in pressure ratio and you are probably right since you obviously know more about this than I do.

I see now that PPS (max/ave pressure in expansion stroke) is perhaps the final limiting factor to raising cylinder pressure to diesel levels, notwithstanding other issues such as very high charging costs, control issues, etc.

So you are most likely right that pressure before ignition cannot be as high for gasoline engines as diesels, as I wrote. It is definitely unlikely that it would be feasible. I was wrong to state that and you are right that it has been a while since I have actually made calculations based on real figures. I am not an automotive engineer (like most of the posters on this forum), but enjoy reading and learning about engine technology on this site, since my career did not go into engine development. I suspect I am not the only one like that posting on this site. But cars are cool and reduced fossil fuel consumption in cars is important for the future - hence our interest.

Peter, I find your debate style a bit too harsh sometimes (not just with me), but I also acknowledge that I have learned something in our present discussions. And thank you for keep coming back to follow up. But please try to be patient with those of us who have not done engine research more than 20 years.

BR
Thomas

Peter_XX,

Now that I think of it, I seem to remember having seen that VW model at a dealership about a year ago... But I forgot about it. I remember being worried about putting such a heavily tuned engine in a heavy vehicle. The engine will be operated at high load much more than in a passenger car, especially if driven by young guys late for work (I see that all the time on the motor ways here in Denmark - black smoke from the exhaust half of the time and brake lights the other half)... I hope they put in a heavy duty gearbox!

The article says in the third last paragraph that a boost pressure of 1.3 bar is developed - 2.3 bar absolute minus pressure loss from the compressor to the cylinder.

A friend of mine has a BMW 135i Coupe with that bi-turbo straigh six... VERY impressive engine. Driving on mountain roads it feels like it does not matter whether you drive it in 3rd, 4th or 5th gear - the acceleration is equally powerful. Oh, that is the gas engine. I have not tried the x35d engine, sadly.