Our #97 mainstream 4-seat Very Light Car achieved 101.4 MPGe in the combined EPA cycle at the X Prize Knockout.
An interesting side note to this accomplishment is that the engine we are using is a 250 cc, one-cylinder internal combustion engine from a Yamaha WR 250R. This small motorcycle weighs 300 lbs and gets 70 MPG, and does not have to meet tough emissions standards.
Our Very Light Car weighs over 700 lbs and gets over 100 MPGe. – while exceeding the stringent 2014 emissions standards, including cold start. How does this leap in mileage occur? Mainly two ways: the extreme platform efficiency of the Very Light Car, and improvements made to the engine. The Very Light Car adds a turbocharger, increases compression, uses extensive exhaust gas recirculation and reengineered internal parts and runs on E85. A similarly efficient gasoline engine we considered doable but would have taken longer, especially the emissions system.
The Very Light Car is not the only entry remaining in the X Prize that can be compared to an existing car’s mileage. Tata’s Indica Vista EV X (a production 4-passenger car, entered in the Alternative 2-passenger Side-by-Side class, presumably to avoid the mainstream 200-mile range requirement) gets 33 – 40 MPGe as a production car, but in the X Prize achieved much more: a combined 134.3 MPGe while running as an electric vehicle.
What explains the much greater efficiency of the X Prize Tata? No doubt it partly reflects the efficiency of the electric motor, properly applied. But this achievement is also a function of the underlying method of measuring energy use in an electric vehicle.
Energy use in an internal combustion engine is straightforward: measure an amount of gas, drive, and measure again. For the X Prize the cars have standardized gas tanks that are removed and weighed.
Electricity use is similarly measured at the X Prize. Batteries are refilled at charging stations and metered for energy consumption: a “plug-to-wheels” formula, that accounts for losses in the charger, in the battery, and energy used by the engine to move the car. Certainly a more accurate representation than measuring the energy use motor-to-wheels, which leads to results all over the map, including some very high mileage numbers.
A problem is that electricity is only an energy carrier and thermal conversion of energy is not considered in this calculation. In the real world for every BTU put into the American power grid for the production of electricity only 52% makes it to the plug. This 48% loss (from energy conversion and distribution) is not accounted for in the plug-to-wheel calculation.
This plug-to-wheel calculation is important as there are many good reasons to shift toward an electric transportation future, such as new options, existing capacity, energy independence and remote emissions. However, a more accurate number for the Tata efficiency would be 40 mpg on gasoline and (134MPGe x .52) 70 mpg as an electric.
This is a more efficient vehicle as an electric but the efficiency comes with costs: battery expense, range restriction and issues like grid capacity and resource allocation. Certainly a path worth pursuing, especially as the electric grid moves away from coal towards renewable sources of energy.
Very complicated issues – what about the efficiency of distributing gasoline or ethanol? - and the X Prize is to be commended for adopting a clear, understandable standard in MPGe.
But the fact remains: our Very Light Car – the most efficient automotive platform ever built – has crossed the 100 MPGe threshold.