The problem with battery electric and fuel cell cars is gasoline. Petrol is genuinely a superior fuel, at least in terms of energy. It has so much energy per gallon, 116,000 btu/gal, that we can use it in an engine whose theoretical maximum efficiency is only 37%. That means that with all the gizmos a modern gasoline powered internal combustion engine has that increase fuel economy, still two thirds or more of the energy in the fuel is being turned into unusable heat, not motive force.
Even with all that waste it still is a more practical powerplant than using batteries or other alternatives. It’s long occurred to me that in addition to improving the efficiency of the engine itself, recovering energy from that waste heat might be a productive way of improving the overall efficiency of the car. Porsche is returning to international endurance sports car racing this year with the 919 that they plan on racing at LeMans, and as part of its hybrid system it incorporates a device that in fact generates electricity from the heat in the exhaust, recovering energy from what would normally be waste heat.
Porsche isn’t the first car company to look into recovering waste heat. In 2005 BMW announced their “TurboSteamer” test bed concept that they said gave 10-15% improvements in fuel economy and power. The TurboSteamer integrated a series of sophisticated (and probably very expensive) heat exchangers into the exhaust and cooling systems to power a two stage Rankin cycle engine that assisted the internal combustion engine. A second generation TurboSteamer was announced in 2011. By then BMW had succeeded in miniaturizing the components enough to fit in the standard packaging of a 5 Series sedan.
One difference between turbochargers and superchargers is that superchargers are mechanically driven by the engine while turbos’ turbines are driven by the pressure and heat energy in the waste gases that are the product of combustion. For that reason, turbocharged engines have greater thermal efficiency than normally aspirated motors. The Porsche 919’s hybrid drive puts a new spin on recovering energy from exhaust gases with turbine devices. The 919’s V4 engine does have a conventional exhaust driven turbocharger, but it has another exhaust driven device that’s part of the race car’s hybrid drive. The 919 is a through-the-road 4WD hybrid. In other words, it has gasoline power driving the rear axle and electric power that can power the front axle on demand. The batteries for the electric motor are charged by two different systems. One is Porsche’s “conventional” KERS-like system that’s used in the 918 Spyder supercar which is essentially a regenerative braking setup. Porsche’s describes the other source of electrons as “a system that recovers thermal energy from exhaust gases via an electric generator driven by the exhaust gas stream.”
What that sounds like is that instead of driving an impeller that pressurizes the fuel/air charge, as a conventional turbocharger does, Porsche’s “fundamentally new” device is an exhaust gas driven turbine that spins a generator. In a sense it’s a reversed setup for an electrically driven supercharger. Instead of a belt drive running off the engine, an electrically driven supercharger uses an electric motor to pressurize the induction. Porsche apparently did the equivalent of flipping one of those around and mounting it on the exhaust system.
We’ll probably seem more efforts, for both racing and road cars, to turn that 63% or more of the energy that’s currently waste heat into usable power. Turbocharging guru Gale Banks, who knows a thing or two about using waste heat energy, recently said on The Smoking Tire’s podcast that he was hopeful that engine waste heat could be recovered by large scale Peltier devices. Peltier devices are what heat and cool your drink in heated and cooled cupholders. They’re very interesting electronic heat exchangers. If you apply DC voltage, one side will heat relative to ambient temperature and the other side will cool. Reverse the polarity and hot and cold change sides. Fortunately, like piezoelectric devices, the entire process is reversible. If you make one side of a Peltier device hot and the other side cold, it will start generating electricity, something called the Seebeck effect. The term Thermo Electric Generator, or TEG, has been coined for devices that use the thermoelectric effect to make power.
Banks hope may actually see the light of day. Purdue professor Xianfan Xu is indeed heading a research team that is developing large scale Peltier devices designed to work with the temperature differentials one finds in a typical car engine and exhaust. BMW is also working on TEGs in addition to their work on the TurboSteamer concept.
Porsche isn’t saying just how many kilowatts their exhaust driven generator puts out, but it’s safe to assume that the power it harvests is worth the weight it adds to the race car or it wouldn’t be there. Race cars may use technologies that are not yet economically practical, but the engineers who design and build them aren’t going to use something that is a net energy loss.
Ronnie Schreiber edits Cars In Depth, a realistic perspective on cars & car culture and the original 3D car site. If you found this post worthwhile, you can get a parallax view at Cars In Depth. If the 3D thing freaks you out, don’t worry, all the photo and video players in use at the site have mono options. Thanks for reading – RJS