he availability of this new air boosting technology is timely as diesel engine manufacturers face ever tighter exhaust emission regulations, which could adversely impact engine performance and especially fuel consumption without the right technical solution,” says Nick Pascoe chief executive CPT. “We now have the means of producing the cleanest engines ever by delivering very quickly and precisely the mass of air necessary for optimum combustion, particularly during transient events, which helps avoid the need for expensive exhaust after-treatment. Electric supercharging is a new and exciting and as yet unexploited technology that will help to reduce fuel consumption and emissions, and thereby the cost of vehicle ownership. Cobra deploys robust and reliable switched reluctance motor technology and is easily packaged with the engine.”
The fully integrated electric supercharger including all control and power electronics, which has taken a decade of development and has worldwide patent protection, will be exhibited at the international conference, which is a high level annual event focused on supercharging organised byProf Dr Ing Hans Zellbeck of the automotive technology institute at the TechnicalUniversity of Dresden. This prominent conference for discussing new legislation and cutting-edge technologies is attended each year by passenger, commercial and off-highway vehicle manufacturers and their technical partners including developers of supercharging technologies from the global automotive supply chain.
Cobra is an acronym for Controlled Boosting for Rapid Response Applications. This unique and highly innovative product is a liquid-cooled switched-reluctance electrical machine with a sealed-for-life bearing system, coupled to electronics that are state of the art, 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 60,000rpm, which is more than sufficient to deliver a high level of airflow and boost pressure with a high level of efficiency. And because it is water cooled it has a high level of thermal stability.
Cobra shares many elements of its design with CPT’s turbine integrated gas exhaust recovery system known as Tigers, 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 650oC 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,” says CPT’s programme manager Andy Dickinson. “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.”
Cobra is aimed not only at excavators and bulldozers, indeed all manner of off highway vehicles ranging from farm tractors to forklift trucks, but also heavy goods and passenger vehicles. In fact almost any vehicle exceeding 3.5 tonnes gross vehicle weight including on highway trucks and buses. Below this GVW the technology has already been acquired by Valeo as a supercharger for cars and light commercial vehicle applications.
According to Dickinson the same critical factor of fuel consumption also applies to developers of big truck engines: “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 NOx reduction, should that become unavoidable with Tier 5 legislation, then Cobra can help without ruining fuel economy.”
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, thereby avoiding cost and complexity. 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 impellors and ever higher rotational speeds currently reaching 200,000rpm thereby adding control complexity as well as significant manufacturing costs,” says Dickinson. “Cobra can operate with a bigger impellor 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 optimised for engine displacements between 4 and 7 litres for which it is ideally matched,” says Dickinson. “It provides engine developers with the means to independently control the air getting to the engine. It’s a powerful tool they’ve never had before.”
Cobra could enable a 5 to 10 per cent 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. 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 NOx control, and can also provide the air necessary for after-treatment systems and burners.
“We believe it’s ideally suited to off road applications, where engine developers have had to resort to expensive VGT solutions to meet Tier 4 emission standards and possibly selective catalytic reduction to meet pending Tier 5 regulations. Electric supercharging provides an additional cost-effective technology for controlling emissions without increasing fuel consumption, which is often the effect when using other technologies. Black smoke in particular which is simply the result of too much fuel and insufficient air can now be significantly reduced or even completely eradicated.”
Cobra’s windings have been optimised for 24 volt operation and the system can be applied to road vehicles exceeding 3.5 tonne and all off highway vehicles. The design specification provides a peak shaft power of 5.6kW 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 500kg/h. The cooling system is integrated with the engine cooling system and the motor will be tested in this environment and validated between minus 40oC and plus 120oC. 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, which have only recently become commercially viable. 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.