I’ve done alittle reading on engines used on old aircraft, like from the 40’s to 50’s, like the DC-7. The engines used a turbine in the exhaust stream to turn a shaft that geared on to the crank shaft, or gearbox, something like that to recover the otherwise lost energy from the exhaust.
Is there any reason that no car or truck manufacturers have done this to increase efficiency? Only thing I can think of is that aircraft spend much of the time at middle power settings (60-70%) where there is more exhaust energy to extract, where cars or trucks spend most of the time at lower power settings? Is this why?
What you’re describing is called a **turbocharger **and they’ve been used on cars & trucks for decades. Part of the reason they’re not used as much on cars & trucks is as you said, because they function better at a steady RPM. Other reasons include:
[li]As opposed to automobiles, aircraft also have issues regarding air density at altitude that make turbochargers more useful.[/li][li]Turbos are high-precision and expensive pieces of equipment (they run at very high RPMs, over 10,000) that require more attention & maintenance than is usually expected for consumer autos (but not for aircraft).[/li][li]You don’t get something for nothing, the back-pressure of a turbo offsets some of its fuel-efficiency & performance gains. Again, this is less of an issue for aircraft.[/li][/ul]
BTW, there is also a similar device called a **supercharger **(or blower) which functions similarly but instead of being powered by exhaust pressure it’s driven by a belt off the crankshaft. Both aircraft & autos use them, but they are much less common than turbos for autos. They give a significantly higher performance boost than turbochargers.
Think about it, a turbo uses energy from the exhaust. A supercharger uses energy from the driveshaft. Now, free energy doesn’t exist, but one of the above is closer to it than the other. Which is why the only manufacturers making supercharged engines are Bentley and Mercedes-Benz, at least those are the only 2 I can think of. And I believe those two use the label ‘supercharged’ and ‘kompressor’ more for historical/marketing reasons than for pure performance or efficiency. That why you’ll never hear about f1 cars or 50 ton trucks being supercharged.
The OP seems to suggesting the exhaust turbine connects back in to the main shaft, That is not a turbocharger which uses the the energy to compress the fuel/air charge prior to injection into the cylinder.
And to Hail Ants, turbo charging has been standard technology for years on diesel engines, both cars and trucks. Small petrol engines (1.2 to 1.4l) are regularly boosted to give power outputs of far larger engines.
Lots of other companies use superchargers in various cars; Ford uses a supercharger in the Mustang GT500, GM uses them in the Camaro ZL1, Corvette ZR1 & CTS-V, Jaguar/Land Rover use superchargers in a number of products (and will be replacing their naturally aspirated 5.0 v8 with a 3.0 liter supercharged v6 in many of the others soon); other parts of the Volkswagen Auto Group besides Bentley use superchargers; Audi has been replacing their v8s with a supercharged v6 too, and so on.
No, a turbocharger compresses the fuel/air mixture. The op is talking about something that works like a gas turbine, where the exhaust gasses drive a turbine that is directly connected to the crank. The extra performance is gained with the same fuel consumption, where as a turbocharger increases fuel consumption for its extra performance. jz78817 is right in calling it a turbo compound engine. Here is the wiki page on it. I hadn’t heard of a turbo-compound engine before, interesting stuff, but fairly rare so presumably not the best way of doing things. The wiki entry suggests that the turbine section of these engines were so efficient compared to the reciprocating part, that they were supplanted by pure turbine engines.
Wow, you’re right. I’d never heard of a turbo-compound engine before! Like every kind of turbine engine the main reason they’re not suited for cars is because turbines are designed to run at a constant (and rather high) RPM and power band, and automobiles are simply not driven that way. Just the opposite in fact. It’s also the reason that, battery weight/power issues aside, an electric motor is probably the best prime mover for stop & go transportation (i.e. cars).
weeeellll… that’s true for a self-contained gas turbine (i.e. one with its own gas generator section.) In this case, the compounding turbine is not the prime mover, and the gas to drive it comes from another engine which is the prime mover. So it’s not quite the same as a gas turbine engine running at lower than peak power.
I think the real reasons it’s not more common is cost and complexity.
I looked over your link pretty carefully and although they used the words “turbo compounding”, as near as I can tell they’re using a conventional turbocharger in a conventional way. (I would love to be proved wrong, because turbo-compounding is cool.)