Gasoline engines have gotten incredibly good at squeezing all the energy they can out of gas. The problem is that "incredibly good" still means about 37% efficiency. If my high school transcript is any measure, 37% still kind of sucks. The rest of that energy is lost as heat, but there are ways to reclaim it. Ask Porsche.
Porsche's 919 Hybrid racer is using a pretty novel drivetrain: a 500HP 2L V4 engine (a first, if you don't count their flat-fours as a 180°V) and an electric KERS system with an electric motor active on the front wheels. But what makes it interesting is how that KERS system is charged up, at least in part: with energy generated via recovery of heat from the exhaust flow.
Engine builders have been using exhaust flow for years to recover power via driving turbines — and the 919's engine does have a conventional, exhaust-flow driven turbo. But it's also using that flow of exhaust gas for another purpose. Here's what Porsche says:
Two different energy recovery systems harness energy to replenish the batteries and provide power. The first system is the innovative recovery of thermal energy by an electric generator powered by exhaust gases. The second hybrid system is a motor on the front axle utilizing brake recuperation to convert kinetic energy into electric energy. The electric energy is then stored in water-cooled lithium-ion battery packs and when the driver needs the stored power, the front motor drives the two front wheels through a differential during acceleration. This gives the Porsche 919 Hybrid a temporary all-wheel drive system, because the gasoline engine directs power to the rear wheels, just like the 918 Spyder.
So, in addition to using that exhaust flow to drive a turbo compressor, they're also using it to spin a generator to produce electricity to store in the KERS system's LiON battery, which can then be employed as needed. This supplements the normal brake-recovery energy systems and should go a long way to keeping that battery charged.
There are some questions about this method — I wonder how it's affecting exhaust flow issues/back pressure, and how they're dealing with that? The bigger question I have is wondering why they're not experimenting with a thermoelectric generator as well?
Thermoelectric generators essentially can turn heat into electricity using something called the Seebeck Effect, and these have been known about for quite a while. During WWII, the Soviets even made a kerosene lamp-powered thermoelectric generator to power people's radios. Porsche (and many other companies) has been experimenting with these sorts of generators on exhaust systems since the 1960s, and by the 1980s they had a Porsche 944 fitted with a thermoelectric generator that made 10 watts.
You'd think that when it comes to getting the most out of reclaimed heat, this would be a way to go — there's no physical impedance to the exhaust flow like there is with driving a generator, which is a big plus, though I suspect the big minus might be that currently, these things are only about 5% effective. Even so, companies like VW have been experimenting to see if such systems can supplement or eventually replace a car's alternator.
There's other ways to reclaim exhaust heat as well — BMW's Turbosteamer collects as much excess heat from the exhaust and cooling system as it can and uses that heat energy to drive either a steam piston or turbine connected to the crankshaft for an extra 14HP or so— not too shabby.
Porsche's 919 Hybrid is the first real use of an engine using reclaimed heat to generate electricity for a drive system, and I predict that as pressure to get absolutely every bit of power and efficiency out of an engine increases, we'll soon see thermoelectric generators on cars, helping out as much as they can.
Until then, if you want to enjoy some free benefit of waste engine heat, just turn your heater up full blast. That's free heat energy right there!