The future of combustionSeptember 24, 2007
Despite the optimism over the electrification of the automobile, I predict that we will be using the internal combustion engine (ICE) in some form or another for many more years. For one, not everyone will be able to afford a pure EV or even a PHEV, and others may find that their needs simply do not match what the EV/PHEV market has to offer. However, for those who equally accept the ICE and the PHEV, this is actually a rather exciting time because the unique environment of the PHEV gives the ICE a chance to shine like it has never before.
1) Advanced valve timing
Given the different physical properties of burnt and unburnt fuel, engineers have for years implemented valve timing schemes that reach a compromise for the engine’s efficiency and power at different speeds. More advanced versions (VTEC, VVT, MiVEC) are able to switch between a “high” and “low” setting depending on the speed. These settings are still inherently compromises, however, and fall short of providing the ideal intake/exhaust timing for most speeds.
However, whether in a series or parallel configuration, a PHEV’s engine (or genset) is necessarily tuned to run at a fixed, narrow range of speeds. Given this unique setup, valve timing can be simplified to take maximum advantage of the single required speed (or just a few speeds if need be), thus doing away with one significant element of added inefficiency, weight, and complexity. The camshafts themselves contribute immense frictional resistance to the engine, and more efficient electromagnetic controls might become possible in the future. These have actually been researched for some time by companies like Valeo, and the simplified operational regime of a genset could make it easier to implement.
2) Advanced turbocharging
Turbochargers (turbine-driven superchargers) use built up pressure and force from the engine’s exhaust gases to force more air into the system, thus improving overall power and efficiency. Thus, the engine needed to drive a generator or the wheels can be reduced in size – which both lowers weight and improves efficiency – while still producing the same power provided by a larger, naturally-aspirated engine. However, there is still the problem of turbo “lag” – waiting for the exhaust pressure to build up after starting from slower speeds.
As an example, the Opel Corsa’s 1.0 liter, 3-cylinder engine produces just over 58 horsepower – which is enough to turn a large alternator or maintain vehicle speed at 55-60 mph with reasonable efficiency. However, it’s very poor for acceleration and slower speeds. Adding a turbocharger for the Corsa would not improve things significantly either, as there would be excessive turbo “lag” along with added power. However, compact, powerful electric motors are proficient at both speed AND acceleration. If one simply unhinges the engine from the wheels, the turbocharger would be much more beneficial than before, as there would be little or not lag due to the constant exhaust pressure. Furthermore, if 58hp was all that was required, the engine could be downsized even below 1 liter, thus further improving efficiency. With little dependence on the engine for acceleration, the turbocharger could also be made larger.
This technology uses high(er) compression and sophisticated fuel injection to achieve improved efficiency over spark plug ignition. The advantage? It works, giving diesel-like efficiency. The disadvantage? It’s complicated. It requires constant computer monitoring to cope with changing engine speeds, and current prototypes have to switch to conventional ignition at higher rpm. This technology is still in its infancy, but the simplified task of a PHEV engine could bring it to market sooner than expected. Once again, the constant speed of the engine would make the creation of a road-worthy HCCI engine a far less complicated task.
4) Novel engine cycles
Various novel alternatives to reciprocating engines exist – such as microturbines and various external combustion ideas – e.g. advanced steam engines, the Stirling Cycle, etc. One limiting factor for many of these engines has been lack of versatility. For example, the stirling engine has good efficiency but poor power. Microturbines with recuperators have good efficiency, but experience this only at very high rpm. Both these designs didn’t make it to mainstream automotive use because they simply could not replace the reciprocating piston engine’s relatively good compromise between efficiency and power. Isolating the duty cycle for an engine’s ideal operating range serves to make it relevant where it would not be before. Cost issues abound, however, especially for microturbines. Small, optimized reciprocating piston engines will likely deliver more cost effective solutions for a long time.