Fundamentals and Empiricism in Engine Design

[kevm14]

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.

That's just it - nothing was going to be wasted. The exhaust comes out at some velocity/pressure (being pushed by the exhaust behind it) and you are sticking a turbine in front of it and making the engine push that exhaust through to spin the turbine. It is an added restriction. And the conversion from flowing gas to spinning turbo happens at some efficiency (below 100%). Then the compressor side pressurizes intake air at some efficiency (below 100%).

Now in my father in law's truck, we could technically stage an experiment. Drive 100 miles down the highway with the engine in normal operation. And maybe make sure there are hills to get into the throttle a little. Record fuel economy. Then, open the wastegate completely and repeat. My hypothesis is that the fuel economy will be better when the turbo is out of the loop as much as possible. It is not a perfect test because the wastegate plumbing itself is still more restrictive than the exhaust would be if there were no turbo at all (plus the intake side still sucks through the turbo, unless you re-plumb that as part of the test), but it's the closest thing I can think of.

And it kind of makes sense. If you are running closed loop, so air fuel ratios are constant, and you are running a constant load (constant horsepower) why would you get better fuel economy with the turbo making some boost pressure? You are doing all that conversion and forcing exhaust through the turbine when the engine could expel the exhaust naturally and ingest air naturally to make that power. Even if the energy to spin the turbo were 100% free, why would an engine be more efficient breathing through a turbo? All a turbo does is force air into the engine. You still need to add fuel.

But that's not even the controversial theory. What is maybe less cut and dry is to take engine A, a turbo 4 cylinder putting out 250 hp at peak, and engine B, a naturally aspirated V6 putting out 250 hp at peak. Which uses less fuel to make 250 hp? All of the BSFC data you can find online says the N/A V6 will use less fuel to make 250 hp. The turbo 4 may be more efficient at making, say, 45 hp, which is why it may get better fuel economy, because it spends more time making 45 hp than 250.

[kevm14]

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

But that still-expanding exhaust gas would much rather take a more direct route to the atmosphere than being forced through a turbo. Which is why it adds backpressure. In fact I think turbos add quite a bit of backpressure. Let me see if I can find some data.

[kevm14]

From a power standpoint, Bill had a thought that if you injected water into the hot exhaust, would the resulting steam/expansion basically add boost? I think the result would be the same as holding the wastegate shut longer, or I guess using a bigger turbo. Recall that all turbocharged applications use a wastegate to modulate boost. You don't just hook the turbo up and whatever exhaust goes through it is what boost you make. You are always limiting the exhaust through the turbo at a certain point or you could damage the turbo.

I think it would also create backpressure - the same pressure that exerts force on the exhaust turbine would press against the exhaust coming out of the head (and, ultimately, the rising piston). Bill mentioned some stuff about a check valve but I think that becomes unfeasible.

Still, I am curious if this has been attempted and what the result was.

[bill25]

There at least has been talk of this:
http://forums.tdiclub.com/showthread.php?t=230043

[kevm14]

A lot of our discussion was covered.

[kevm14]

New information...many folks measure their exhaust back pressure and compare it to intake (boost) pressure. Ideally, you'll have a 1:1 match. In reality, and especially in street cars, you'll have more back pressure in the exhaust than you have boost in the intake. For example, 20 psi boost may be 30 psi in back pressure. One more reason for the extra fuel usage in boost compared to an N/A engine of the same power (lower efficiency). And wrapping the exhaust...that will increase back pressure (keeping the heat and thus pressure in the exhaust). Which makes/helps the turbo work harder, but certainly doesn't add fuel efficiency. This is all very interesting.

[kevm14]

More info on back pressure from the Sloppy Radio Show ep6.

For street cars, in his experience (with V8s), it is normal and acceptable to have a 1:1 ratio of back pressure to boost up to ~14psi and creeping up above 1:1 on the way to 20 psi boost and beyond.

He also stated that on one bulls he saw 20 psi of boost with only 18 psi of back pressure, which he considered to be very good.

What is the point again? The point is the engine pumps out exhaust against this pressure. It is not free energy.

[kevm14]

https://m.youtube.com/watch?v=1xbK4F7QdPM

Quick vid showing a surprising amount of interesting things.

At the bottom is air fuel ratio which is rare to hear about in turbo diesel applications. Easy to see where the efficiency comes from - they run quite lean at part throttle.

But look at boost (top) vs back pressure (middle). As I have said for forever, turbo add substantial back pressure to the engine which is pumping loss. Here it looks to be around 1:1 at heavy throttle.