Use of a Low-Cost Electric Supercharger Could Significantly Reduce Smoke from Turbocharged Diesel Engines
|CPT electric supercharger test rig. Click to enlarge.|
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 under certain operating conditions by the addition of a relatively simple and low cost electric supercharger, suggests Controlled Power Technologies (CPT).
Controlled Power Technologies (CPT) is a UK startup working with switched reluctance motor technology to deliver cost-effective CO2-reducing solutions for vehicles. It is commercializing three initial products: Variable Torque Enhancement System, VTES, an electric supercharging system; SpeedStart12, an integrated belt-driven starter generator system; and TIGERS, an exhaust turbine-driven generator system. (Earlier post.)
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. 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 minimize the need for expensive exhaust after-treatment.—Nick Pascoe, chief executive officer CPT
The benefits of having a near instantaneous air supply can be achieved by CPT’s VTES. Twin sequential turbocharged solutions are unable to match the faster mass flow rate rise and corresponding torque response delivered by VTES at low engine speeds, CPT says.
|“VTES...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 70,000 rpm 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 minimizing 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.
|VTES in a sample emissions control application. Click to enlarge.|
In a fixed speed torque step at 1,250 rpm, test data taken from a 3-cylinder 3-liter diesel engine revealed a reduction of the area under the smoke curve of 89% and smoke opacity reduced from 8.3 to 1.4%. 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. Traditional high pressure EGR systems result in poor driveability, which is an issue that can also be addressed with VTES technology.—Guy Morris, Engineering Director
Pascoe, accompanied by Morris, will discuss the technical issues of the use of the supercharger system with other delegates attending the 5th International Commercial Powertrain Conference, organized by AVL at Graz in Austria this week.
|Fully integrated VTES unit. Click to enlarge.|
VTES. VTES is an air-cooled Switched Reluctance machine, coupled to electronics and an optimized radial compressor, that delivers high airflow, pressure and efficiency.
The product is designed for integration into both Otto and Diesel engines to deliver enhanced torque, emissions control and CO2 reduction. CPT has done a range of studies on the application of VTES, including:
- A current, small (<1.6L) naturally aspirated Otto engine;
- An extreme 1.2L turbo Otto;
- A 2.0L turbo Otto for light-duty trucks; and
- A mid-range diesel for light commercial vehicles.
VTES is optimized to use the standard 12V vehicle architecture.