Emissions: An engine apart
Running a combustion engine independently of the wheels gives engineers more scope to optimise emissions but CO2 isn’t always top priority
- Published in Focus.
Moving on: Opel Ampera’s simple four-cylinder engine makes a low-cost range extender
Internal combustion’s role in supplying society’s demand for personal mobility is beginning to change. With hybrids beginning to gain significant market share, consumers are getting used to the idea of the piston engine being stationary even when the vehicle is moving.
That concept should become a lot more familiar when the Chevrolet Volt goes on sale in the US next year, followed in Europe by the Opel Ampera. These extended-range electric vehicles are meant to provide the average consumer with a vehicle to get them to work and back without the crankshaft completing a single rotation.
Designing internal combustion engines specifically for the role, where maximum engine speeds are lower, the range of operating points much lower and cold starts far less frequent, also gives engineers new opportunities to optimise emissions.
Lotus Engineering is developing a three-cylinder gasoline engine specifically as a range extender. The 1,193cc monoblock design features an integrated exhaust manifold, two valves per cylinder, a 10:1 compression ratio and a simple port fuel-injection system. “With a clean sheet of paper to develop a range extender you might think that a diesel would be desirable but our assessment was that it’s not,” says Jamie Turner, Lotus’s chief powertrain research engineer. “Operating diesels at peak efficiency means a poor NOx-particulate trade-off.
“Aftertreatment is expensive and you frequently have to pull the engine away from peak efficiency to regenerate it. This makes diesel an expensive solution and its overall efficiency may not be much better than a gasoline engine operating in its area of peak efficiency.”
Lotus’s range extender always runs stoichiometrically, even on the motorway. So like most spark ignition engines it only requires a simple three-way catalyst to clean up the tailpipe emissions.
Because the engine isn’t mechanically linked to the wheels there’s greater freedom to optimise the start-up procedure in terms of speed and load and therefore the mass airflow through the engine. This affects raw emissions and therefore catalyst loading so the precious metal content could be reduced slightly.
“Once you’ve determined your strategy you can heat the catalyst very quickly,” says Turner. “So there’s the potential to de-content the catalyst a little, although three-way cats are pretty cheap already.”
