Do I understand correctly that a turbocharger puts less load on the engine than an equivalent supercharger, because it is driven by exhaust pressure rather than a mechanical linkage? If so, why does the same not hold true for driving an alternator or air conditioner compressor?
Alternator/HVAC power requirements aren’t necessarily in lock-step with the power that’s available from the exhaust stream at any given moment. OTOH, the power requirement to compress intake air is very closely coupled with what’s available in the exhaust, so it makes much more sense to use an exhaust turbine to power an intake air compressor. Thus, the turbocharger.
One other option is turbocompounding: instead of using turbine power to operate an intake air compressor, you add that power to the crankshaft. In a roundabout way, this power can then be used to operate the alternator/HVAC and other accessories, since the crankshaft nose is what powers all those items.
In F1 racing, heat generated by the turbo/exhaust is harvested and converted to electric energy and stored in batteries as part of the hybrid system. “Motor Generating Unit - Heat” (MGU-H) is working along with the more familiar kinetic energy recovery (braking and spinning electric motor in reverse to brake and store energy).
Many on-road applications are proven on race tracks. So, yes, the exhaust heat is in play, taking the exhaust from just powering a turbo to a new application (recovering energy to be deployed as needed).
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There is also the Comprex, aka pressure-wave supercharger.
one reason is because they’re non-linear. the turbine(1) relies on gas flow to do its work, which is why turbos have traditionally had some degree of “lag” before the engine generates enough exhaust gas flow for it to spool up and build boost. If you’re just puttering off down the road, the engine might not be delivering enough gas flow for a turbine to be able to power accessories like A/C. It’s much more predictable (and easier) to have the alternator and A/C driven by the engine, and just switched on and off when needed.(2)
plus, since the primary purpose of a turbocharger is to increase the engine’s volumetric efficiency (VE), you can afford the moderate increase in restriction in the exhaust due to the turbine/housing. The compressor being driven by the turbine is increasing the engine’s power capability by quite a bit. A turbine driving loads like A/C would just present a drag on the engine’s power as it now has to force the exhaust through a(nother) turbine which isn’t helping increase the engine’s VE.
(1) It’s worth remembering that a turbocharger is more or less a gas turbine, but one which needs a separate engine as its gas generator
(2) modern cars have some surprisingly complex strategies on how and when it energizes the alternator’s field. back in the day the voltage regulator more or less tried to keep the electrical system at a stable 13.8-14.4 volts. now, the PCM controls the alternator field and will decide when to energize it based on demand and battery state of charge.
Slight hijack, but this thread reminds me of another way to take off power for accessories - engine vacuum. I had an old Ford Bronco with vacuum powered windshield wipers. Worked well at idle, not worth a damn while on the gas. In heavy snow you had to manage the throttle for both road speed and wiping ability.
Not really addressing the OP though. Carry on.
Don’t bother trying to get mechanical energy as a commodity out of the flow of exhaust from the engine. That’s what the whole engine is for. That’s the main job pistons and crankshafts and connecting rods DO. Everything else is to get that to happen.
OK, that makes a lot of sense, thank you!
Yes, but that part would be true for a shaft-driven supercharger, wouldn’t it? But I can see that a turbocharger might ramp up a lot more at high RPM than a shaft-driven system, a characteristic that would be desirable for a super/turbo-charger but not for other accessories.
Thanks. I would imagine that this is more practical for aircraft engines that operate at more constant RPM than automobile engines?
that gets into cost/benefit. it adds quite a bit of complexity and there’s a trade-off between using the turbo’s turbine to provide mechanical output or more air to the engine. There is a truck engine which uses it, the Detroit Diesel DD16 but I don’t know of any others. Detroit Diesel also doesn’t say how much horsepower it adds.
I imagine that for cars, right now the costs of turbo compounding aren’t yet justifiable. if anything, if it was to show up I’d think it’d be on pickup trucks first.
missed the edit window:
well, here’s the thing- superchargers are tied to the speed of the engine, you have to weigh the tradeoffs of where in the engine’s rev range it does its “best” work. being driven by the engine is fine for accessories like alternators and compressors; the output of the alternator is controlled by the field current so as long as the engine is spinning it sufficiently fast (e.g. anything above idle) it can supply the needed power. A/C compressors are controlled by engaging and disengaging an electromagnetic clutch, so again as long as the engine is running the compressor can do its job.
Turbochargers, on the other hand, are tied to engine load. the amount of boost they provide depends on the volume of exhaust gases being delivered to the turbine. They can provide peak boost at rather low RPM (unlike superchargers,) which means that they also improve the engine’s low-end torque*. this is basically why you wouldn’t want to power accessories via an exhaust turbine; you’re basically never running your engine at high load so the turbine would not be able to run the accessories most of the time.
- this is why the whole “diesel=torque” thing is a bit of a myth. diesels are immune to detonation, so they’ve used high-pressure turbocharging for a long, long time. It’s only since the advent of gasoline direct injection that gas engines have been able to use similar turbocharging strategies. It’s not diesel which gets you torque, it’s turbocharging.. a gasoline turbo direct injection (GTDI) engine is capable of just as much torque as a diesel of similar displacement, and a hell of a lot more horsepower.