Turbos and fuel efficiency

"Ron Calls on his years of experience....and freezes at the controls" (internerdstuff) 02/13/2015 at 09:43 • Filed to: None

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Hi guys.I'm not exactly sure how to word this question so i'll just give it a try and see where it goes.Sorry if i reply slowly- being on the butt end of the world adds an inevitable delay ;-)

By now everyone and their dog knows turbos are the new IN thing for increasing fuel efficiency. The usual reason given is that a turbo engine can be smaller and thus has less friction losses. That i can understand easily. However as turbo engines need to have lower overall compression ratio,doesnt that mean they have less 'decompression' to make power from? What i mean is- arent turbo engines essentially opposite to stirling cycle engines which have a low compression ratio and high decompression ratio?

Also, obviously turbo or supercharged engines can also be stirling cycle types as well(where they are then usually called miller cycle). Is it perhaps a normal thing for modern efficient turbo engines to be at least partly stirling cycle. Could *THIS* and not the turbo as such,be the main factor in their efficiency?

RamblinRover Luxury-Yacht > Ron Calls on his years of experience....and freezes at the controls 02/13/2015 at 09:56

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Smaller/less friction losses counts less than you'd think - there are additional losses imposed by a turbo. Inertial losses in rapid decel/accel somewhat more so. The main factor to efficiency is actually that they are *in effect* capable of variable displacement. Varying the boost allows you to have (effectively) a small engine when needed *or* a large engine when needed. This is important because short of weird combustion chamber management you need to be burning a certain amount of fuel to a certain amount of air at a certain speed for a given displacement, regardless whether the car *needs* it. If you get to vary the amount of air for that displacement instead, you can vary the fuel, and the power produced by it to be only what you need.

That being said, a very efficient larger NA engine up until last gen could still turn out roughly as good real world numbers because of lower parasitic losses in some operation.

Sejji > Ron Calls on his years of experience....and freezes at the controls 02/13/2015 at 09:57

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I always thought it was because when you're crusing around 2k rpms you're not throwing much boost into the engine (less fuel & air). This combined with a smaller displacement will increase your fuel milage numbers. It's a low strain on the engine to maintain speed from 55-70 mph. When you're running a car at full boost and acceleration, the economy numbers are actually lower than a NA engine, because you're cramming more air and fuel into there (the higher power figures have to come from fuel in the end). No one measures full acceleration fuel economy numbers, so they get to post the max HP numbers, and avg. fuel numbers simultaneously.

That's how I always rationalized it. I know it' isn't a super detailed engineering analysis.

Will with a W8 races an E30 > Ron Calls on his years of experience....and freezes at the controls 02/13/2015 at 10:06

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This is a typical BSFC map:

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It shows engine load (essentially torque/liter) on the y axis, and engine speed on x. The lower the number in the island, the lower the engines fuel consumption per HP. As is pretty obvious from the plot, the engine is most efficient (in terms of mpg/hp) at high loads and low rpm. It is gets inefficient quickly as you go faster, due to a number of factors such as friction and pumping losses.

In the EPA fuel economy test, and in standard driving (normal person), the engine spends most of its time at low speed. If the engine is large relative to the vehicle, this will also be low load, and therefore, inefficient. If you have a tiny little engine, it will be working hard at those normal low speed points, and operating in the efficient low speed/high load region.

Based on that, the most efficient car would be something with a small engine kept at low speed and near wide open throttle, rarely exceeding 3500rpm. But that small engine is not very powerful, and when you really step on it, and let it rev, the result is going to be disappointing.

This is where turbocharging comes in. Take that efficient tiny engine, and put a turbo on it. On the EPA cycle, or with a sane person driving, that little turbo engine will stay out of boost, and the engine will operate around that nice efficiency island like the efficient NA car describes above. But when the turbo spools up, it raises that red line at the top of the map - when you get into boost, the max load of the engine increases, and it has the power that you expect. It's no longer efficient, but you're also not driving like grandma or the EPA cycle expect you to. The engine makes plenty of power, and when you're done driving like an asshole, it goes back to being that undersized, fuel sipping grandma engine.

That's the theory anyway. In practice, drivability issues make those characteristics difficult to achieve smoothly, and you end up getting into boost a lot earlier than would be ideal to try to eliminate turbo lag. Things like twin scroll turbos can help with this, but there are still compromises.

BiTurbo228 - Dr Frankenstein of Spitfires > Will with a W8 races an E30 02/13/2015 at 11:41

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What a fantastically concise explanation. Impressed :)