Turbocharger (Physics of Fuel Efficiency)

[Good4you]

I have a general understanding of how superchargers and turbochargers work; however, there is something that I have always had a hard time conceptualizing as a physics problem.

Super/Turbo chargers add horsepower. This makes sense as they essentially cram more fuel into the combustion chamber in the same amount of time. Typically super chargers are thought to be less fuel efficient than the same engine naturally aspirated, which makes sense because they rob horsepower off the belt to drive them. Turbo chargers I usually think of as more fuel efficient. My first question is a bit more of a practical question then physics related, and doesn’t really effect my main question.

1) Are turbo charged engines more fuel efficient than the same motor naturally aspirated, or just more fuel efficient than a naturally aspirated motor producing similar horsepower (assuming the naturally aspirated one has a larger displacement, and therefore more moving parts and friction losses?)

a. I assume diesel vs gas, makes no difference; though for some reason diesel’s seems to have much more benefit from turbos?

Ignoring the car (aerodynamics, rolling resistance, etc.) it seems to me that the fuel efficiency of a motor would be pretty straight forward conceptually. How much power can a given amount of fuel produce (energy density), and subtract out some heat/friction losses to run the motor, and some losses for not fully combusting the fuel (I’d be curious what those %’s are.) Exhaust plays a role in the losses (that’s why larger exhaust/mufflers provide better fuel economy.) So the exhaust side of a turbo provides some exhausts resistance and more losses to power the intake side. Conceptually I would think that whatever losses you got on the exhaust side, would balance out whatever gains you get on the intake side. It makes sense that a turbo gives you more power because even if you can’t get as much power for the same amount of fuel due to friction losses, you can still go through more fuel in the same amount of time. But this doesn’t explain the fuel efficiency.

2) Conceptually why would a turbo be more fuel efficient?

Mechanics might say you are re-capturing wasted energy from the exhaust pressure, but this doesn’t really explain it for me, because the process of re-capturing creates its own losses. It seems a bit like trying to strap a generator to a wheel and have it power your car motor. Whatever gains you hope to get from the generator are more than offset by the friction it adds. Obviously this example is similar to how a hybrid car works, but the difference is that a hybrid recaptures what would normally be lost in braking due to friction/heat from the brake pads, so you are not getting gains during powered driving (right?) (e.g. city mileage in hybrids are a lot more impressive than freeway mileage.) I know a hybrid has some other helpful bits to get better fuel efficiency, like always running the engine at its optimum engine speed. I think the answer to my question might have something to do with the engine speeds, and power curves that change with a turbo, but I cannot seem to connect the dots in my head. Or maybe I am way off base. Any insight would be appreciated.

 

[Good4you]

Put more simply:
If you think of a turbo as a stand alone system it is essentially two fans connected by a shaft. It converts mechanical energy from exhaust gas to mechanical energy for the intake. The system must have a net loss or it would be an impossible perpetual motion machine. How does adding a system with a net loss to another system (the engine) not make the engine less efficient? Shouldn't the turbo losses take away from the efficiency of the engine?

 

[rcgldr]

Are turbo charged motors more fuel efficient than a naturally aspirated motor producing similar horsepower (assuming the naturally aspirated one has a larger displacement, and therefore more moving parts and friction losses?

The smaller turbocharged engine is more efficient at low power output, when the turbocharger isn't doing much. Eaton is making superchargers that have clutch mechanisms, and/or designed to produce less boost and consume less fuel at lower rpms. Once at moderater or high power output, where the turbocharger assist is significant, then the milage is about the same, its' the overhead of the losses in the turbocharger stuffing more air to go along with more fuel, versus the overhead of a larger engine that produces the same power. An alternative would be to make an enginge of similar size, but could operate at much higher rpm, but the efficiency at lower rpm and lower power output would suffer, the cost of such an engine would be higher, and most consumers would not like to drive a car with an engine that constantly runs at higher rpms. High rpm engines are common among motorcyles, but considering their weight and aerodynamic drag, they are high performance with relatively low fuel milage around 35 mph, less than some small cars.

 

[SteamKing]

Put more simply:
If you think of a turbo as a stand alone system it is essentially two fans connected by a shaft. It converts mechanical energy from exhaust gas to mechanical energy for the intake. The system must have a net loss or it would be an impossible perpetual motion machine. How does adding a system with a net loss to another system (the engine) not make the engine less efficient? Shouldn't the turbo losses take away from the efficiency of the engine?

You're getting mixed up here. Yes, turbochargers, like every other machine, are not 100% efficient. But the overall efficiency of a turbocharged engine is not calculated by adding the efficiency of the naturally aspirated version of the engine to the efficiency of the turbocharger.

The turbo runs off the energy contained in the hot exhaust gas produced by the engine. In the NA version of the engine, this energy goes out the tailpipe and is lost. By adding a turbo to the exhaust, you increase the efficiency of the engine in two ways:

1. Some of the energy from the exhaust drives the turbo and the attached compressor, which means that you are extracting usable work from the exhaust which would otherwise go out the tailpipe and be lost. It also means you don't have to use some of the engine's power output to turn a belt to drive the compressor.
2. The compressor driven by the turbo is able to force more air into the engine to be burned with the fuel, avoiding the intake losses which occur in a NA engine. The air comes out of the compressor at a higher pressure than ambient, so the engine does not have to work as hard to compress the air/fuel mixture in a SI engine (or just the air for a CI engine).

 

[Dylanqoneal]

if it helps at all, maybe just food for thought, MY subaru 2.5L N/A has the same fuel efficiency as the WRX which has a 2.0L turbocharged engine. so to dumb it down i believe its SMALLER ENGINE + TURBO = SLIGHTLY BIGGER ENGINE. but there are so many other variables to it all considering a turbo engine has a different compression ratio and all. also the PSI added helps atomize the fuel molecules to it burns better while in the combustion chamber, thus creating a more efficient engine. idk, just some stuff i know about it.

 

[Kozy]

Essentially the fuel burnt is always linked to the power. A typical value of BSFC would be around 260g/kW/hr at peak power. So if your average 2 litre performance engine can make 200bhp (149kW) at 7500rpm it will consume 38.74kg of fuel an hour.

A turbocharger works to reduce the BSFC by allowing power to be made at lower RPM and with a smaller engine, both of which reduce the friction.

A 1.6 turbo might be able to produce 200bhp (149kW) at 6000rpm. The smaller bore has less ring/wall friction, the smaller surface area leads to reduced thermal losses and the lower RPM leads to less friction in the crank bearings and also in driving the ancillaries.

End result of all these little gains combined is that the 1.6T might be able to muster a BSFC of 250g/kW/hr so in that same 1 hour at full chat it would use 37.25kg of fuel.

 

[jack action]

A piston engine is basically 2 machines combine into one: A device that convert heat into mechanical work and an air pump. The supercharger basically only affect the air pump part of the engine.

In a NA engine, the air pump is represented by the intake and compression stroke, which is a piston pump. This is very efficient as whatever energy that was used to compressed the air is returned on the power stroke (even if there is no combustion) with very few losses (since everything is done in the same space, there are no losses associated to fluid transfer). It just cannot be beat efficiency-wise. The quantity of air that can be compressed is linked to its volume and its rpm. Unfortunately, it prefers larger volume and low rpm.

When you add a supercharger, you pre-compress the air before entering the air pump. The advantage is to use a pump type that prefers working with small volume at high rpm. Keeping everything equal, you will of course reduce the compression of the «engine» air pump (by lowering the mechanical compression); but the air is still compressed globally to a similar compression ratio.⁽¹⁾ The problem is that these compressors are most likely to produce, globally, a less efficient pumping phase than the traditional NA engine. First, because of all the fluid transfer that is done; second, with the mechanical losses through the linking of the 2 machines (belt, gears, etc.).

The exception here is the turbocharger. It takes its energy from the wasted heat in the exhaust system. The turbine kind of does the opposite of what I just explained: The turbine «increases» the power stroke of the engine to extract more energy from the high pressure gas, just like the compressor can «increase» the compression stroke of the air pump. This is where engine efficiency can be increased.

But turbine efficiency is very sensitive to the mass flow rate and this efficiency gain can rarely be seen throughout the operating range of an engine.

One big advantage of pre-compressing the air is the possible use of intercooling. This allows to increase the overall compression ratio without inducing pre-ignition and thus cramming more air-fuel mixture into the engine. A slight efficiency increase will also be gain that way, which might offset all other losses, such that the supercharged engine might have fuel efficiency very similar to the original NA engine.

This is why efficiency gain are mostly related to engine sizing and weight reduction as rcgldr explained in post #3.

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(1) A gain can be obtained by reducing the engine mechanical compression ratio: It increases the volume of the combustion chamber, which makes it easier to give it the best shape to lower heat losses and promote flame propagation. This is probably why turbocharger has a more prominent effect with high-compression diesel.