Thoughts on turbocharger behaviour

Julian · Post by **Julian** » July 7th, 2006, 1:10 pm

The turbo was specified to operate at a nominal peak of 1.6bar - a long way from 2bar. Because we are using the VAG controller there is a degree of waste - in effect we run the turbo below what would normally be peak efficiency (it is an awful lot more complicated than that of course) and as such the controller can be turned up to use more of the wasted boost. The engine doesn't like it though and is not really configured to utilise the potential.

The engine is being rebuilt with another turbo but I have no been involved with the project for quite some time and the rebuild is out of our hands.

Julian · Post by **Julian** » July 7th, 2006, 5:24 pm

Seems I've been guilty as the next man on those performance figures. Managed to find a printout from a dynorun - while it doesn't list the various temperatures and pressures it does list a peak recorded power of 226bhp at 6000rpm but the graph doesn't show any signs of the power or torque dropping off.

Given the run was limited to 6000rpm I suspect this is one of the early runs from before we sorted out some niggles so I still stand by my original statement but I can't back it up right now. I will try and get the later run graphs from the owner (if he is talking to me).

SteveNZ · Post by **SteveNZ** » July 8th, 2006, 2:51 am

What engine are you reffering to and what turbo does it have?

Julian · Post by **Julian** » July 8th, 2006, 8:34 am

I am referring to the mule engine. The turbocharger is (or rather was) a custom built unit based around a Garret T28.

SteveNZ · Post by **SteveNZ** » July 8th, 2006, 10:08 am

Um, whats a mule engine?

Testament · Post by **Testament** » July 8th, 2006, 10:12 am

SteveNZ wrote:Um, whats a mule engine?

As in a test engine, "test mule" for experimentation etc.

Julian · Post by **Julian** » July 8th, 2006, 10:40 am

My apologies Steve - yes a "mule" is a test bed for development. In this case the first installed engine as a result of the development work. The first engine never made it out of the dyno cells and nor was it intended to. The whole point was to test the proposed engine configuration to destruction. The only two things we discovered from the development was that 1) we had underspec'd the ring landing on the pistons (this could have been a fault in the manufacture of the pistons as only one failure occured and it was very minor); and 2) the oil pump drive shaft is prone to failure over 12000rpm (and like that was a big surprise....).

The mule engine performed excellently once the niggles were sorted (I still want to find the later dyno sheets and not the early one as I'm sure the figures I have came from when it was limited to 0.8bar) but the final engine will be further modified to use a dry sump arrangement - in effect covering all of the bases and pre-empting the lateral loadings the engine will be subjected to.

SteveNZ · Post by **SteveNZ** » July 8th, 2006, 10:54 am

So what is this mule engine, Uno turbo?

Tee · Post by **Tee** » July 8th, 2006, 12:30 pm

I understand (I think) Testament's logic regarding the bleed valves and compression ratio reduction. However, I also understand that racing applications are mostly about peak power/performance not efficiency. I prefer efficiency, but I'm noone. Wether that is a personal preference or a result of how I was educatued so far, I can't say for sure, but I suppose that racing is more of a "whatever it takes" type of philosophy, or so in a nutshell.

I still think about the 1.5L BMW engine that was claimed to do over 1000bhp. I would love to know more about that.

Regarding matching problems, I'll go out on a limb here, but I believe it is good to express this to the people in the field so they can help me understand this better, I know I'm going to be slammed for this for all eternity but I'm prepared to take the chance....

What hurts me is that, at times, I have to reduce specific engine performance (modify cam timing, reduce compression ratio, retard ignition...etc.) away from optimum, which means that I often times loose engine performance, so that when the turbo is added total performance is improved safely.

The matching problem gets very tricky when the engine needs to be modified (mods mentioned above) to run the turbo. I tried in my spare time, but I have no experience. However, which is more important? The turbo? Or the engine that is actually making the power? Or it doesn't matter as long as the total is better than non-turbo? (whatever it takes) The last bit sounds like the 8.0L 400+HP V10 Viper wins over the 100+HP/Liter most of Europe currently aims for, or the 240HP 2L Honda S2000. Turbos increase efficiency, I know that. But loosing power to gain power feels like running around in circles.

Unfortunately, the philosphy behind the matching problemse scapes my ignorant mind, not the problems themselves, I understand the reasons for the existence of a matching problem. Throughout the simesters, I have studied that turbocharging improves efficiency (simple, just draw the cycle diagram). But what hurts me is the trade off where I have to "yield" the engine to "the turbocharger's demands", or to put the engine in a "safe zone" when the turbo is installed.

It's easy to think of the turbocharger exclusively and assume the engine is just a complex combustion chamber and run the cycle as if it is a jet engine (brayton/gas turbine cycle). For sure the gas turbine cycle has it's advantages as a source of power, but the engine is what drives the car, not the turbo.

I feel that the main requirement (which is the engine) should not be yielded. I'm not saying the gains are not real, not at all (note BMW engine above, which is fantastic), nor that lowering the compression ratio is wrong, so far it is necessary in order to run the engine with a turbo on it with a certain level of reliability. It's a real world necessity as far as I can think and tell. But the engine runs better with a higher compression ratio anyways, let's boost that instead, keep the engine at peak performance/efficiency and then turbocharge it. I think higher octane and better intercooling should be the answer, nothing else. I would like to see a turbocharged S2000 engine. No mods, stock engine + turbo.

I am still studying for my degree...so go easy on me. :D

SteveNZ,

I think the "mule" is a 1.4L UT engine.

Julian · Post by **Julian** » July 8th, 2006, 1:47 pm

Tamer,

Don't forget that the Formula 1 engines of the turbo era were 1.5L 4 cylinder engines. The output of these engines was peaking at around the 1200bhp mark - a seemingly fantastical figure but one that is recognised and verified.

Tee · Post by **Tee** » July 8th, 2006, 1:56 pm

Perhaps "fantastic" was not a good choice in wording...my fault.

What I meant was, wonderful, excellent, marvellous, etc... :D

sumplug · Post by **sumplug** » July 9th, 2006, 9:56 am

remember chaps, a turbo is just an air pump. it denses the air so that the cylinders fill with a bigger charge for a bigger explosion, so more power and torque. getting the static and dynamic compression right is important. the shape of the chamber, ports especially around the valve is always important. the main thing to do though is make sure the cylinders scavenge quickly so they can be refilled.!! if you keep the flow up, then both power and torque benefit. i would rather have a high flow engine with low boost, then a lowish flow engine with high boost. the latter is always what most do. they turn up the boost or fit a bigger turbo, and fit bigger injectors, and then wonder why the engine is very laggy or the bottom end is all over the road when it goes bang!!
at the end of the day, the turbo and engine are glorified air pumps. get the flow right through the engine, build in strength and relaibility,and dont go for silly boost figures and all will be ok!! :D

Julian · Post by **Julian** » July 9th, 2006, 10:53 am

Ultimately you have a good point but the behaviour of the induction system is radically different. The engine is no longer sucking - except off boost - instead the turbo is pushing the gas in faster than the engine can pull it. This makes a change to the way you go about designing the ports - yes you still need to keep the valve area clean in the same way as you would normally. The rest of the system defies traditional design methods and you can often see better performance by just cleaning up the casting.

The point about having a high-flow, low-boost engine is correct but not the entire story. This is where the step of matching is very important. The selection of turbo package is important. Just bolting a bigger turbo on steps outside the desired performance envelope unless you select a package that will work with the engine. I highly recommend going to have a read of the tech basics on the garrett website (http://www.turbobygarrett.com/turbobyga ... ch101.html is the first one). The bit you should be really interested in is the third section (http://www.turbobygarrett.com/turbobyga ... ch103.html) which goes into explanining compressor maps and efficiency islands. This is what you should be using to select the correct configuration. When most people just bolt on a larger turbo they are paying no attention to where the efficiency islands are in comparison with the engine's flow performance. Get it wrong and the whole thing just fails to gel.

Post by **Guy Croft** » July 9th, 2006, 1:59 pm

Hi

some good points being made by all - well done.

GC

formerly
Chief Engineer Product Support
Napier Turbochargers, Lincoln

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Thoughts on turbocharger behaviour

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