Low cost electric supercharger eliminates smoke from diesel engines
Smoke emissions produced by turbocharged diesel engines under heavy load conditions - such as when accelerating from low engine speeds and other similar transient manoeuvres - can be all but eliminated by a relatively simple and low cost electric supercharger says automotive CO2 reduction specialist Controlled Power Technologies (CPT).
The reduction of particulate emissions is a major issue for the off-highway segment of the commercial vehicle industry, which faces tough new international regulations in 2012, when for the first time legislators will assess the emissions of engines under transient conditions. Until now the emission testing for such engines has been steady state.
"The particulate problem is caused by the turbocharger being sized for maximum power rather than transient response," says Nick Pascoe chief executive officer CPT. "During a transient event, the fuel injection system tries to deliver the power demanded by injecting more fuel, but the turbocharger will be a few seconds behind, with the lack of air causing over rich combustion and lots of soot. A fast-acting electric supercharger helps with the root cause of the problem by supplying more air earlier in the event. It can also help minimise the need for expensive exhaust after-treatment."
Pascoe, accompanied by engineering director Guy Morris, will have the opportunity to discuss the technical issues with other delegates attending the 5th International Commercial Powertrain Conference organised by AVL at Graz in Austria this week. The two-day forum, held on 28-29 April 2009, provides a platform for industry experts and automotive engineers responsible for trucks and buses, agricultural tractors, non-road commercial vehicles and industrial machinery to exchange ideas.
The benefits of having a near instantaneous air supply can be achieved by CPT's electric compressor known as VTES or ‘variable torque enhancement system'.
The technology provides an attractive solution for the industry because of its low cost, low effort of application engineering and low commercial risk. This is particularly the case when compared with twin sequential turbocharged solutions, which are unable to match the faster mass flow rate rise and corresponding torque response delivered by VTES at low engine speeds.
"VTES is a compact system with fully integrated electronics that's easy to install," says Pascoe. "It dramatically increases the engine air charge density over the first few critical combustion cycles of a low speed transient operation, enabling real improvements in both torque and emissions performance."
Unlike mechanical superchargers and exhaust gas driven turbochargers, VTES operates independently of engine speed. Its low inertia compressor accelerates from idle to a maximum of 70,000rpm in less than 350 milliseconds. The system is highly dynamic and the intake charge boosting and resulting torque enhancement is achieved very typically in less than 250 milliseconds.
As well as avoiding the need for a twin sequential turbocharger, the VTES approach can help reduce the size and cost as well as minimising the uncertainty in regeneration strategies of using a diesel particulate filter. VTES effectively provides an additional control that can be used during vehicle calibration to reduce particulate emissions and increase transient torque.
In a fixed speed torque step at 1,250rpm, test data taken from a 3-cylinder 3-litre diesel engine revealed a reduction of the area under the smoke curve of 89 per cent and smoke opacity reduced from 8.3 to 1.4 per cent. Over the NRTC test cycle the associated smoke emission is virtually eliminated. The NRTC test is a transient driving cycle for mobile non-road diesel engines developed by the US EPA in cooperation with European Union authorities.
As well as reducing particulate emissions, VTES technology can also be applied for NOX control. As part of a low pressure exhaust gas recirculation system it can deliver sufficient EGR flow even with a low pressure difference across the engine and rapid purging of EGR in the ducts and intercooler during high load transients.
"Future emission controls will require increased use of exhaust gas recirculation even under higher engine load conditions," says Morris. "Traditional high pressure EGR systems result in poor driveability, which is an issue that can also be addressed with VTES technology."
CPT's VTES supercharger benefits from almost a decade of research and continuous product development resulting in a robust and cost-effective system.
Formed as a management buy-in funded by private venture capital, the company is backed by a number of prominent investors specialising in the energy and environmental sectors. CPT's understanding of power electronics and control software and how electrical machines can be applied to vehicle powertrains probably best defines its area of expertise.