Engine technology thread.

[serialk11r]

Okay after a bit of research and thinking I think this is definitely possible. A turbocharger not attached to the engine usually would run at higher speeds, but mechanically linking it to the engine would change up things a bit. Ideally we'd want CVT connecting the 2, but I believe a 2 speed gearbox is probably good enough to get near peak efficiency most of the time.

A single speed link would probably be okay though. Garrett's compressor maps seem to indicate 200k rpm as the maximum allowable speed. Of course having the turbine spin at close to maximum speed isn't best because losses tend to be the greatest, but a chart I found on the BorgWarner website seemed to indicate peak efficiency at around 160k rpm, slightly lower at 180k, and climbing up consistently from lower speeds. That said, the 80k curve was still in a pretty healthy zone, within 20% of maximum efficiency, but if we extrapolate a bit we can see that the efficiency drops off quite a bit at lower speeds. This is probably why turbo-compounding has appeared in diesel engines first: they have only a small rev range to worry about, so the turbine doesn't need to operate at inefficient conditions.

If our engine has a 8000 rpm redline though, things seem rather inconvenient. With 200000rpm maximum turbine speed, we want a 25:1 gear ratio to make full use of the "useful" operating range of the turbine. At say 750rpm idle though, this is a pathetic 18000 rpm at which the turbine does barely any work, and at 2000 rpm, we are only at 50000rpm at the turbine, at which it can only achieve perhaps 50% of max efficiency. Certainly with higher gear ratios for fuel economy, if we are running 2500rpm on the highway, our turbine does not operate very efficiently. However while cruising and idle, there isn't as much exhaust either, and the pressure ratio across the turbine would be close to 1 anyways. So that brings us to acceleration. But when we get over 3000rpm the turbine can do pretty well. So for city driving, we probably want something more like a 35:1 ratio to make the turbine useful from 2000-4000rpm. But then again, you can stick to one ratio and rev a bit higher in each gear.

As far as turbo sizing goes, the Garrett website does some approximate calculations with a 2L engine that is supposed to make about 400hp. It would need about 24psi boost to do this at 8000 rpm (not intercooled though I think, but just using this for quick computations). In my previous post I used the fact that there is perhaps more than 3atm absolute pressure left in the cylinder when naturally aspirated. Let's say there's about 30% of the energy in the pressure difference that can be captured. (so use 0.75atm gauge pressure to calculate). This is about 15hp from what I said previously, so we want the turbine from a turbo with compressor that uses 15hp under peak power conditions, which would be a turbo that you'd use to generate about 7psi boost. These calculations are super super super rough, but that gives you sorta an idea of the difference between sizing of the turbines between just a turbocharger and a turbo-compound (with supercharger or not). With say 5psi boost we will have something like 30% more exhaust pressure as well, so the turbine should be from a turbo that makes about 10psi boost, and so on. However you'd want to use a smaller aspect ratio turbine housing since you're not using as much air.

Oh and the more boost you add the more useless this turbine will be at part load, of course. On a racing car though this would provide pretty significant benefit in power.

Another side note: I am honestly not too sure if manufacturers would ever consider this as an efficiency increasing measure, when it's possible to add displacement and use late intake valve closure instead, although late intake valve closure is a bit tricky to do valve timing output control with. Of course more displacement has its own issues too...I guess if it came down to cost, 25% more displacement is cheaper to do than a turbo compound setup most likely, but then with more displacement you'd run into the issue of possibly overexpanding the exhaust at part load. Dunno...

[old greg]

Originally Posted by serialk11r

EDIT: Oh and I forgot to mention, someone has done this with a Mazda 13b adapted for use on an airplane I think.

I was just reading about that a few weeks ago. Have you seen this?
http://www.rotaryeng.net/why-tc.txt

The problem with turbocompounding a production car is that the turbine won't do jack squat at low engine loads. It needs a lot of pressure, which is only produced when you're letting lots of air/fuel into the cylinders. Look at the places turbocompounding has been applied commercially, aviation and big marine engines are both operated a near full throttle at all times.

Now, putting one on a race car does make sense, especially if you're running in a displacement, restrictor or fuel limited series. If you're only allowed a 2L engine or say, a ~26mm restrictor, free horsepower is a big deal. And if you're in an endurance race, reduction in SFC at WOT is also a big deal.

As for your mention of lag, there won't be any. Once you couple a turbocharger to the crankshaft you have a centrifugal supercharger and the power band that goes with it. On the plus side, you won't have to worry about wastegates.

[serialk11r]

Wait I did say there was no lag...
So IMO the use for this on a race car is 1. no lag 2. get the most out of what boost you can run, aka get more horsepower without needing to upgrade internals. Point 2 is kinda why anyone would possibly think of doing it to a street car. With a street car I believe you can argue a turbo is equally useless, why not just a supercharger? Good low end torque, poor fuel consumption only when you're really on it (and as I pointed out a turbo doesn't actually save that much energy).

[old greg]

Originally Posted by serialk11r

Wait I did say there was no lag...

I must have misread.

Originally Posted by serialk11r

So IMO the use for this on a race car is 1. no lag 2. get the most out of what boost you can run, aka get more horsepower without needing to upgrade internals. Point 2 is kinda why anyone would possibly think of doing it to a street car.

Perhaps we have different definitions of the term "race car". If you're talking about somebody's modified production car that is no longer street driven, turbocompounding is a very poor investment. The development of such a system will cost a lot more than a set of forged pistons, and probably a set of rods as well. And since you're talking about running a supercharger anyway, it's much, much cheaper/easier to make up the extra power the old fashioned way.

Actual race cars (ie, cars that are raced against other cars under a common set of rules with tech inspections to make sure that everyone is being honest) are limited in the amount of power they can produce so that the racing stays close and exciting (and relatively safe). This is usually done with some combination of intake restrictors, fuel capacity, displacement limits, and boost limits where forced induction is even allowed. This is where turbocompounding actually makes sense.

Take FIA Formula 3 for example. All F3 cars must be fitted with a 26mm intake restrictor, which means that no car has more than ~215 HP regardless of how huge the team's budget is, the laws of physics prevent it. Recovering 15 hp of waste power from the exhaust gasses means that you could show up on the grid with 230 and have a big advantage. Of course, that's not to say that the rules for any given series don't disallow it (either specifically or on a technicality like F1's rule against variable geometry exhausts), or that it wouldn't cause a rules change for the next season or even the next race. But advantages in racing are hard to come by.

Originally Posted by serialk11r

With a street car I believe you can argue a turbo is equally useless, why not just a supercharger?

Because apples are better than oranges, duh.

[82mm 4g63]

Originally Posted by serialk11r

With a street car I believe you can argue a turbo is equally useless, why not just a supercharger?

You can believe anything you want, doesn't make it reality.
See religion.

[serialk11r]

Sure, currently it's not easy to develop such a system because large capacity aftermarket turbochargers exist at relatively low cost, but there's nothing stopping it from being financially feasible! Essentially all that needs to be done is rethink the way the turbine is sized compared to the compressor, along with housings, and attach a gearbox to the turbo shaft and then couple that to the engine via a belt or chain. Maybe in the future automakers will turn to this sort of idea to improve volumetric efficiency, and we'll have plenty of standalone power turbines sitting in cars for this purpose

@above post: I believe meant "you can definitely". With a turbo the only "extra" energy you can get is from the velocity of the exhaust gas, except a single stage turbine doesn't collect very much of this impulse. The efficiency advantage of a turbo comes at higher boost levels and higher altitude, where the difference between cylinder pressure at exhaust valve opening and atmospheric is greater. In addition using a blow off valve is probably worse than the bypass valves built into superchargers as far as efficiency is concerned. I have to say, madfast's turbo complaints got me thinking about the actual advantage of a turbo, and they are slightly more efficiency under some conditions, better for high boost, and easy to create peaky power down low.

[82mm 4g63]

Originally Posted by serialk11r

@above post: I believe meant "you can definitely".

You can definitely...what? Were you referring to my "above" post? Or greg's?

[Dimman]

Serial:

There is an article on turbo-compounding in either Race Tech or Racecar Engineering magazine (can't remember which, I buy both) for this month or the last month.

Talk about it coming in to LeMans and maybe F1. They show 3 different ways to use it.

Simple exhaust drives a turbine which attaches to the crank via reduction gears.

'Normal' turbo system with the above system to further scavenge energy. I think they said that it needed to have a certain amount of boost to create enough of a pressure differential for it to really work.

And a supercharger system with it attached.

They talk about its history in airplanes and how Cummins and Caterpillar are using it in big rigs for fuel savings.

I'll have a re-read tonight, and try to give a better synopsis than this half-remembered gibberish tomorrow.

I originally thought you were talking about staging turbos (one feeds into another before the engine) when you were talking about compounding.

[serialk11r]

Originally Posted by 82mm 4g63

You can definitely...what? Were you referring to my "above" post? Or greg's?

Yes your post. You can definitely argue a turbo is silly on most cars because a supercharger does a similarly good job in a much simpler way. I forgot to mention the energy that backpressure costs the engine cannot be fully recovered by the turbine, so it's actually somewhat vague whether a turbo is actually more efficient than a supercharger. My guess would be with most exhaust manifold and turbine housing designs which don't utilize the impulse of the exhaust pulses very well, there probably is very little advantage if any at all.

@Dimman: Good to see other people are thinking about this too In the second application that's definitely right, you need quite a large pressure drop to make a dual stage turbine (essentially what it is) worth it over a single stage turbine. With a small supercharger just one exhaust turbine stage would probably be pretty good. You can also add more turbine stages and gear them appropriately, as some of the aircraft engines did in the past I think, although using a turbocharger in conjuction with a second turbine for power generation would probably be simpler and better. Application 1 would be just to get a bit more energy out of every drop of fuel in a (high compression) naturally aspirated engine, upping power a little bit.

[Marrk]

Originally Posted by serialk11r

Yes your post. You can definitely argue a turbo is silly on most cars because a supercharger does a similarly good job in a much simpler way.

Didn't MINI go from Supercharger to turbo on their S model? What do you suppose the thinking was on that?

Just wondering.

[serialk11r]

So if you look at what a lot of people are saying they want in terms of forced induction, most people are saying they'd be happy with just a modest amount of boost, like 0.5bar. The Mini runs up to 1.3bar boost; it has lowered compression, BMW's twin scroll turbo, etc. In addition the torque peaks very early. I think the point of the car is more to be good for street driving and fuel efficiency (it has a 1.6L engine afterall). BMW also threw Valvetronic on it, which tells you how serious they are about getting very good fuel economy (Valvetronic pushes it in favor of turbo because now at partial loads where the turbo does spool a little we don't have to waste this with the blow off valve, but this boost can be used to reduce the amount of compression the piston has to do).

I think for people who want to add just a bit more power to the FT86, a supercharger is probably a good idea. At lower levels of boost, the extra amount of pressure going out the exhaust is not that great. One way to say this I suppose is a turbine operates better at higher pressure ratios, but if you don't have much boost, the pressure ratio in the exhaust is not going to be very good, and the turbo won't help you that much. From the research I did yesterday I think it seems like aftermarket turbo suppliers don't make twin scroll housings for smaller turbos like this one, which further dilutes the turbo advantage. See what I'm getting at? If you just want to add a little bit of power the extra hassle for turbo might not be worth it.

[82mm 4g63]

Originally Posted by serialk11r

Yes your post. You can definitely argue a turbo is silly on most cars because a supercharger does a similarly good job in a much simpler way. I forgot to mention the energy that backpressure costs the engine cannot be fully recovered by the turbine, so it's actually somewhat vague whether a turbo is actually more efficient than a supercharger. My guess would be with most exhaust manifold and turbine housing designs which don't utilize the impulse of the exhaust pulses very well, there probably is very little advantage if any at all.

There is nothing vague about it. You're comparing the back pressure cost of a turbocharger to the total crankshaft power lost by a supercharger? There is no comparison and no argument. Roots/Lysholm/Eaton blowers can lose as much as 1/3 of the total crankshaft power output, show me a turbocharger that looses anywhere close to that (don't bother looking, you won't find one.) Best case scenario, supercharging would be AS efficient as a turbocharger (and I'm being more than generous), but still won't compare in peak power.

IMO the only plus to supercharging is it's lag-less power band, but when you take a modern, ball-bearing turbocharger and a driver who knows how to drive a turbocharged car you won't have any lag complaints. Sure a supercharger is simpler, but when that simplicity is a direct result of it's most significant drawback (power loss) how can that be considered a plus? It would be like saying, I just lost 50 pounds, but I had to chop off my leg to do it.

Superchargers are the masters of lag and installation simplicity, while turbochargers are masters of efficiency and offer more power.

[82mm 4g63]

Originally Posted by serialk11r

So if you look at what a lot of people are saying they want in terms of forced induction, most people are saying they'd be happy with just a modest amount of boost, like 0.5bar. The Mini runs up to 1.3bar boost; it has lowered compression, BMW's twin scroll turbo, etc. In addition the torque peaks very early. I think the point of the car is more to be good for street driving and fuel efficiency (it has a 1.6L engine afterall). BMW also threw Valvetronic on it, which tells you how serious they are about getting very good fuel economy (Valvetronic pushes it in favor of turbo because now at partial loads where the turbo does spool a little we don't have to waste this with the blow off valve, but this boost can be used to reduce the amount of compression the piston has to do).

I think for people who want to add just a bit more power to the FT86, a supercharger is probably a good idea. At lower levels of boost, the extra amount of pressure going out the exhaust is not that great. One way to say this I suppose is a turbine operates better at higher pressure ratios, but if you don't have much boost, the pressure ratio in the exhaust is not going to be very good, and the turbo won't help you that much. From the research I did yesterday I think it seems like aftermarket turbo suppliers don't make twin scroll housings for smaller turbos like this one, which further dilutes the turbo advantage. See what I'm getting at? If you just want to add a little bit of power the extra hassle for turbo might not be worth it.

Define "a bit more power" please? Do you mean like 50whp? With gains that modest twin-scroll is overkill. If you absolutely wanted to go twin-scroll the smallest and most affordable one you'll find would probably be off an Evo4-8. They came with factory twin-scrolls, but they also had titanium alumnide alloy rotating assemblies so they spun up 500rpm sooner to boot. If the turbo has an R(twin-scroll) and an A(titanium) in it's name you've found a winner.

However, if all you want is 50whp and twin-scroll isn't a must have, get a little GT25R(~250whp) or GT28R(~310whp) from Garrett. I know ATPturbo sells them.

Also, the lack of availability has no effect on the "turbo advantage." Just because even the smallest turbochargers offer significant performance increases, doesn't make turbocharging a bad idea. Twin-scroll is primarily used for larger turbochargers, it's pretty pointless in smaller ones because they already spool so fast as it is. The most you'll gain is a couple hundred RPM on a 2.0L engine. For example, look at a Hyundai Genesis Coupe dyno chart.



That's a pretty responsive little factory turbo (Mitsubishi TD04H-13TK3) and it is neither twin-scroll nor ball-bearing.

[serialk11r]

Perhaps in the real world it works better in some way, but if you just examine the thermodynamic cycle, and consider a turbine that works purely off a pressure drop, the piston has to supply most of the energy powering the turbine. The turbine has some losses, so at the end of the day we don't have much of an efficiency improvement.

If you draw out a P-V diagram for yourself you'll see what I mean. It's very important that the turbo collects the kinetic energy of the exhaust pulse wave to end up more efficient than a plain old supercharger (acting as an impulse turbine). If the exhaust manifold and turbine housing are not optimized, you have a definite loss in efficiency. The point of twin scroll appears to be faster spooling, but dividing the exhaust pulses improves its function as an impulse turbine (and also acts to scavenge exhaust gas from cylinders better), which is probably why BMW put a twin scroll on a 1.6L mini that only makes barely over 200hp.

What's more is as I said the turbine is always sitting in the exhaust stream, and the only method to relieve backpressure is to vent with the blowoff valve. However the blowoff valve itself has a large pressure drop. A supercharged engine with bypass valve or clutch simply acts like a naturally aspirated engine at low loads while the same engine with a turbocharger will always experience additional backpressure as described. I don't doubt for a moment that under heavy load a turbo is more efficient, it's just a turbo has some disadvantages a supercharger does not have, and that the advantage is not as big as people might think.