Fundamentals and Empiricism in Engine Design

[kevm14]

Oh yes.

Also some good discussion here.

http://forums.autosport.com/topic/10593 ... n-engines/

Bill and I were having a debate on why forced induction engines have higher BSFC than naturally aspirated.

[kevm14]

Yay, this is what I was saying before.

At the moment, the best way to obtain "efficiency" in a SI passenger car turbo application is the technically false but nonetheless practical type of efficiency obtained via the "dual use" concept: a small-displacement engine for economy in normal use, coupled to a turbo for greater performance on occasional demand

You could, however, make up specific applications/situations where there are more benefits than just being able to use a smaller engine.

For example, comparing a 600hp turbocharged LQ4 and a 600hp naturally aspirated LQ4-based build. It would seem to me that the turbocharged LQ4 would get better light load fuel economy simply due to the required cam (being non-variable) on the N/A build. The engine just wouldn't be efficient at lower engine speeds. The turbocharged variant could run a much more mild cam and thus be more efficient at light load/cruising situations.

The 600hp N/A engine, however, still would be more efficient when outputting 600hp, at least based on...maths. And thermodynamics.

[kevm14]

More explanation:

Meanwhile, neither the exhaust turbine nor the compressor turbine are anywhere close to 100 percent efficient. So in the true and final test of efficiency, BSFC, turbos are better than crank-driven superchargers but not as good as normally aspirated. The energy required to drive a turbo to significant effect is not "free" but in fact an additional pumping loss. This has been proven enough times to be downright depressing.

He does say turbos are better than crank-driven supercharges, and for part load fuel economy, that does seem to be quite true. But the rest is what really sheds some light. Any time you convert energy, you ALWAYS have a loss. So let's see what we have so far:
- Turning waste heat into spinning a turbine is not a 100% efficient process, nor is the compressor on the other side turning rotational energy into pumping air
- Turbos run richer air/fuel ratios, though direct injection likely mitigates this significantly
- Turbos add back pressure so the volumetric efficiency of the engine does go down when it is diverting exhaust through the turbo (i.e. the wastegate is closed)
- Forced induction engines generally run lower compression than N/A which is an efficiency reduction when cruising (less of an issue with DI)

The formula for deriving economy benefits was already summed up in my last post (the quote). The only thing efficient about a turbocharged engine is, perhaps ironically, when you have the wastegate open and are bypassing the turbo.

The rules completely change for turbo-diesel engines, which have no air throttle. BECAUSE they have no air throttle. They actually get more efficient since they make the engine overall more volumetrically efficient, but power output is controlled by the injection of fuel. And for the same amount of fuel, you can get more energy out if you have forced air in there (a leaner mixture).

[kevm14]

Here's a theory on why the BSFC (i.e. full load) of supercharged engines is said to be higher than turbocharged (even if turbos have the advantage at part load):
Because converting energy from something mechanically driven probably has fewer overall losses than the dual conversion loss of the turbo, which also adds back pressure. Maybe?

[kevm14]

I missed another key paragraph:

As a result, one of the more illusory statements in automotive technology is that turbos operate on "wasted heat energy." Not really. In practical terms, one is better off regarding a turbocharger as simply a pump. You can put a turbo on the bench and heat the turbine wheel to white hot with a torch and it will not budge. To make it rotate with any authority you are going to need some pressure. In terms of "blowdown energy" it is helpful to remember that the typical automotive engine can be stopped in its tracks with a potato. In the old piston aircraft industry when the Wright Turbocompound was developed, "blowdown turbos" and "pressure turbos" were regarded as two very different beasts.

I will at least say that this jives with my understanding. I mean, if the inertia of the exhaust spun the turbine, that would be something (like hydroelectric power) but that's not the case.

[kevm14]

Not to cloud the discussion but as I said, turbodiesels ARE a totally different beast:

Large diesels are nowadays achieving a good deal of the turbo-compounding efficiency gains simply by running very high boost and returning the work to the crankshaft via negative pumping losses. In large sizes, turbine and compressor efficiencies exceed 80%.

SI engines in steady state are also able to achieve a negative pumping loss situation (boost pressure higher than exhaust BP) so the compressor work is free. The biggest impediment to thermal efficiency for SI engines is the necessity to operate substantially richer than stoich as boost increases. Every drop of fuel beyond stoich goes straight out the exhaust.

Most likely a turbo diesel runs leaner than stoich even under boost, which is nothing you could accomplish in a gas engine, even with DI.

[kevm14]

Bringing this back.

http://blog.autospeed.com/2008/12/04/op ... l-economy/

My father in law is getting close to lighting off his truck with newly added turbo and I've been thinking more about turbo theory, boost control and so on. I mean I have to tune the thing, right?

The thought I had that made me stumble on this article is about fuel economy. As I addressed above, the premise that "turbos increase fuel economy" needs to be dissected to understand what is really meant: a turbocharged engine of smaller displacement than the engine it replaced should get better fuel economy at light load precisely because the engine is smaller. More specifically a smaller engine will require a greater throttle opening at the same power level and thus has fewer pumping losses (and probably less internal friction).

Ok, but what about wastegate control? The wastegate stays closed until boost pressure rises enough to overcome the spring, or until the boost controller sends the vacuum/pressure signal to the wastegate and it modulates boost accordingly. What happens at part throttle? Presumably the wastegate stays closed and if the turbo is sized right to the engine, you get boost at part throttle. This should make the engine drive bigger than it is but I don't think it is good for fuel economy. The article above says some cars actually open the wastegate unless boost is really needed. This increases efficiency because a turbo adds backpressure to the engine when exhaust is forced through it. Also, a turbo trying to force air across a partially open throttle doesn't sound like efficient operation to me.

All of this kind of solidified why they say the BSFC (at full power) of a turbo is the worst, followed by supercharging followed by natural aspiration. I don't think there's anything "free" about putting a restriction on the exhaust of a car (the whole "use waste heat to make power" theory). Just as there isn't anything free about attaching a belt to a crankshaft to spin a supercharger. Except as I mentioned in an older post, the conversion of energy from the crank to the blower is probably more efficient than the way the turbo does it, and the turbo adds backpressure which the supercharger does not. At light loads, the added backpressure is probably not really restricting exhaust flow since everything is sized for peak power, which is why a turbocharged engine may not pay a fuel economy penalty during modest highway cruise, for example.

And back to my father in law's 296cid inline 6 with turbo, I think what may happen is the engine will make more torque at part throttle, but use more gas than before. This doesn't actually matter but it is interesting.

[bill25]

I never thought that the heat turned the turbine. The only way I could even imagine that this would be possible is if there were a secondary state change reaction in the exhaust manifold, like spraying water and creating steam from the heat. I could see this expansion adding pressure to make the turbo spin faster. I wonder if this would be testable.


I basically thought the engine acted as an air pump and it used the wasted exhaust air to turn the turbine. I can see that this would potentially restrict exhaust flow but I also thought that a certain amount of backpressure helped the engine.

[bill25]

This pretty much goes along with what I said about the water injection at the exhaust manifold/before turbo:

By comparison, a turbocharger does not place a direct mechanical load on the engine, although turbochargers place exhaust back pressure on engines, increasing pumping losses.[13] This is more efficient, because while the increased back pressure taxes the piston exhaust stroke, much of the energy driving the turbine is provided by the still-expanding exhaust gas that would otherwise be wasted as heat through the tailpipe.

https://en.wikipedia.org/wiki/Turbocharger

[bill25]

So they briefly mention this but they do it after the turbo, instead of before:

Water injection[edit]

Main article: Water injection (engines)

An alternative to intercooling is injecting water into the intake air to reduce the temperature. This method has been used in automotive and aircraft applications.[