Cobra could enable a 5–10% reduction in CO₂ emissions when implemented in conjunction with engine downsizing and down speeding strategies, and potentially more when used in conjunction with alternative fuels, according to CPT. For emissions control applications, it delivers a very rapid transient air increase for particulate control, a low boost pressure EGR pump opportunity for part-load NO_x control, and can also provide the air necessary for after treatment systems and burners.

Cobra is a liquid-cooled switched-reluctance electrical machine with a sealed-for-life bearing system, coupled to electronics providing a high level of precision and digital control. Combined with a radial compressor connected to a low inertia rotor, the device accelerates within milliseconds to 60,000 rpm, which is more than sufficient to deliver a high level of airflow and boost pressure with a high level of efficiency. Because it is water-cooled it has a high level of thermal stability.

Cobra’s windings have been optimized for 24 volt operation. The design specification provides a peak shaft power of 5.6 kW and a continuous shaft power of 3.4kW; a peak pressure ratio of 1.4; and a peak mass air flow rate of 1,134kg/h and continuous mass air flow rate of 500 kg/h. The cooling system is integrated with the engine cooling system and the motor will be tested in this environment and validated between -40 °C and +120 °C. The power and control electronics are fully integrated and similarly cooled. Without coolant the dry weight of the machine is approximately 6kg. It is typically mounted directly to the engine.

A key enabling technology for Cobra is the switched reluctance motor, which is now ready for the automotive industry and high volume mass production following the development and availability of low voltage (12-48V) electronic components. Switched reluctance motors without the integrated electronics are already deployed extensively in other industries. They are inherently robust and reliable, highly efficient over a wide speed range and avoid the use of permanent magnets and rare earth materials.

Cobra shares many elements of its design with CPT’s turbine integrated gas exhaust recovery system known as Tigers (earlier post), which is currently under development for installation next year in a technology demonstrator part funded by the UK Technology Strategy Board. The Tigers electrical generator is currently running on a CPT engine dynamometer at 650 °C but will eventually run at much higher temperatures.

The aim with Cobra was to develop a 24 volt electrical supercharger to eliminate transient turbo lag, in other words be active for a few seconds or more but sufficiently robust to do so repeatedly without any thermal management issues. Cobra can handle the recurring transients of continuously variable engine loads without degradation, which is underlined by our ability to run it on test continuously and in the same boost mode for 30 minutes or more. So it will cope therefore even with an off-road digger continually demanding variable power and torque from its engine.

Commercial vehicle transport managers and off road operators certainly don't want to be wasting fuel and ultimately it’s always the metric they use for deciding whether emissions technology is cost effective or not. The benefits of Cobra to the engine developer and end user depend on the application. But if they’re running their engines continuously and need additional air for transient performance or exhaust after treatment, for example when purging diesel particulate filters and other forms of exhaust after-treatment for NO_x reduction, should that become unavoidable with Tier 5 legislation, then Cobra can help without ruining fuel economy.

—CPT program manager Andy Dickinson

The torque increase and transient response with Cobra also provides a cost-effective alternative to expensive variable geometry turbocharging by simply combining the electric supercharger with a standard, lower-cost, fixed-geometry turbocharger. Unlike a turbocharger, Cobra is not dependent on the exhaust gases flowing to do its work. Because it’s electrically powered, it can quickly deliver the air required to get the exhaust gases flowing so the turbocharger can then start to do its work. And as it’s disconnected from the engine, electric supercharging can provide an immediate transient response, especially at low engine revs, thereby overcoming one of the fundamental deficiencies of a turbocharger.

With a turbocharger the design engineer has to pursue ever smaller impellers and ever higher rotational speeds currently reaching 200,000 rpm, thereby adding control complexity as well as significant manufacturing costs. Cobra can operate with a bigger impeller which swiftly delivers significantly more air at slower rotational speeds, which also helps with reliability and durability.

Cobra is targeted at diesel engines and for initial applications has been optimized for engine displacements between 4 and 7 liters for which it is ideally matched.

—Andy Dickinson