For the X Prize the Very Light Car is powered by a one-cylinder, 250cc internal combustion engine running on E85. As the name implies, E85 is an 85% ethanol, 15% gasoline fuel blend.
To understand why we chose an internal combustion engine and E85, we need to explain why we did not chose a hybrid or electric drive. Our early studies on efficiency and the X Prize competition showed that car weight can be the main determinant of a vehicle’s energy consumption.
With weight being of paramount importance, the energy content of gasoline vs. batteries comes into play. A 6-pound gallon of gasoline contains the energy of a 500lb battery. For the 200-mile range required by the X Prize for the mainstream class that translates into 1000 lbs of batteries. Our 4-seat mainstream entry weighs under 750 lbs, so the Very Light Car would more than double in weight if electric. That’s huge in terms of overall efficiency.
An electric car has to work extra to accelerate because of the added weight of batteries. All cars require less power to maintain highway speeds than to accelerate up to them, and this is especially true of electrics. An advantage of electrics is that the motor can be throttled back to cruise without losing efficiency.
A gasoline engine may accelerate more efficiently without the added weight of batteries, but when throttled back to cruise power it loses efficiency, because of internal friction and pumping losses.
Pumping losses led us to E85. Pumping losses come about at low load because the engine is trying to draw in a cylinder’s worth of air past a throttle that’s trying to stop it. Our solution for this inefficiency is to “throttle” the engine with exhaust gas recirculation. For practical purposes exhaust gas is inert, so if we send a cylinder full of exhaust gas and fresh air mix into the engine instead of a partial fill of fresh air only we can reduce power while also reducing pumping losses.
However, there is a limit to how much you can dilute the incoming fresh air with exhaust gas before the engine starts to misfire. E85 is significantly more tolerant of charge dilution than gasoline, allowing us to run the engine at close-to-peak efficiency at cruise power.
E85 has other attractive qualities. It (ethanol) runs cleaner than gasoline. It is renewable. As ethanol production moves from corn to cellulose-based sources ethanol becomes more energy efficient to produce and competes less with farmland.
As our last post noted, we are energy-source agnostic. The Very Light Car increases auto efficiency regardless of power source. But for the X Prize, at least, E85 makes a lot of sense.