Turbocharger differences, diesel vs. petrol

[Artsi]

The very dilemma I have, is to gain a better realisation on differences between diesel engine turbocharger characteristics and petrol (gasoline) engine turbochargers.

After some time consuming browsing over various turbo models, end result indicates differences in turbines. Diesel engine turbochargers generally speaking do have larger turbine wheel minor diameter than on the compressor side. Petrol engine turbo models have somewhat equal size minor diameters on compressor- and turbine wheels.
I assume this comes from diesel fuel’s different energy content, which most likely will generate more exhaust volume to play with. Some have said that diesel engines produce around 25-30% more exhaust gas flow than petrol engine. Any confirmation to this?

Does anyone have any resource for determining suitable turbine housing/wheel combo for diesel engine application? Automatic calculators on various turbo shop web pages are always for petrol engine application. Even a ‘rule of thumb’ would do for me.

Any commentary relating to topic (even remotely) is most welcome.

 

[ivymike]

Diesel engines and gasoline (fuels) have approximately the same stoichiometric AFR. Gasoline engines generally run very close to stoich (about 14.7:1). Diesel engines often run in the neighborhood of 27-33:1, although they may be in the low 20's under certain conditions. What expect would you expect that to have on exhaust flow for a particular power output?

 

[MikeHalloran]

Gasoline engines run throttled most of the time, i.e. with the valve inlet at a relatively low absolute pressure.

Diesel engines don't even _have_ throttles; they pass as much air as their displacement allows.

Gasoline exhaust gas temperatures are also typically a bit higher than a Diesel's EGT.

For a lot of reasons, Diesels are also more expensive to repair, and hence to break, so fewer people risk breaking them. There are a few hot rod shops for small truck Diesels, but at this point they'd be silly to put what they know out on the Web for you to learn for free.

Do you have a particular engine in mind?










Mike Halloran
Pembroke Pines, FL, USA

 

[NakedTestMonkey]

Locally I have seen enourmous power output from compound charging. Essentially you run a pair of turbochargers, one feeds the intake of the other effectively increasing boost pressure. If you are looking for sheer peak output that might be something to look into.

Don't overrule trim and housing variations when looking into Petrol vs. Diesel turbochargers. Petrol turbochargers have to be capable of operating out of surge at low throttle and mid throttle points, so the properties of the compressors could carry a number of differences.

 

[MikeHalloran]

I went through a few rounds of trying to design a lightweight marine exhaust system for Paxman Diesel engines in a super- showoff boat. Five and a quarter liters per cylinder, eighteen cylinders, nine turbochargers compounded, 4MW @ 1950rpm. Only three exhaust outlets to deal with, but ridiculously low backpressure limits, and ridiculous expectations about silencing the engines with vestigial mufflers in a tiny engineroom.

Luckily, the engines went out of production before we got sucked further into the deal.

That engine had three LP turbos feeding six HP turbos, with the exhaust of the HP turbos driving the LP turbos.

I don't think I'd recommend compounding turbos until you had successfully engineered a few dozen single stage setups.



Mike Halloran
Pembroke Pines, FL, USA

 

[NakedTestMonkey]

That sounds like a pretty interesting gig Mike.

 

[Artsi]

I thank you folks for a good flow of responses. Let's continue forward.

I am seriously playing with idea of two-stage turbocharging (i.e. turbo compounding, which actually does mean connecting turbo mainshaft mechanically onto gearbox, but let's not get into that today..) my small German made 1.9liter fourbanger diesel engine.

Two-stage turbocharging has been around since the 80's for sure, among tractor pulling scene. From there it escalated onto commecial heavy machinery. Today 2-stage charging can be found on Caterpillar C-series engines straight from dealer floor. Most recent developement in this field on passenger car engines has been by BMW in it's 535d model series. That's engineered by Borg-Warner turbo manufacturer. (Google can find a ton of stuff on that).

Aftermarket tuning shops have mostly catered the needs of large diesel engined pick-up vehicle crowd, as it come to this method of forced induction.

What I'd like to achieve, is to pick up correct size charger to go with smaller unit I have in the engine as standar issue.
What I've studied this, indicates that everyday street driven car benefits from building a 2-stage charging using the original turbo as the latter compression stage. This will aid engine to come on boost at moderate engine rpm.

Competition engineered two-stage charging needs to perform well in the upper rpm range. That'll mean a touch larger turbo units on both, the primary- and secondary stages.

I myself am heavily biased towards making a decent boost over stock rpm range. I want engine to come on boost, come on hard, at low rpm.
I am looking to get 30psi/2bar boost as low rpm as possible, and maintain that to the redline rpm. I know that boost can be achieved with a single charger only, no probs at all. Operational rpm range will be much narrower with single turbo setup, and also at worse compressor efficiency.

Perhaps Opel will follow BMW's way anytime soon with their 1.9liter engine, pls have a look here:

http://www.worldcarfans.com/news.cfm?newsid=2040414.009&opel=1.html/country/gcf

--

A study I read on the subject dealt with a truck engine that had been 2-stage charged. Primary turbo was 117% of ideal size, and smaller secondary stage was 90% of ideal size. (Sizes were compared over to ideally selected single charger setup size.)

The very essence of 2-stage charging is to have high boost out of compressors that are happily working in their modest compressor map sweet spot. Boost will also come on much earlier as comparing over to single turbocharger.

Two compressors connected in series, both pushing 14.7psi/1.0bar boost, will result in total boost of 44.1psi/3.0bar. (Those are not absolute pressure values, they're boost pressure values.)
Pretty neat.

I need to choose the large primary charger from some big diesel engine. What gives me the headache, is that on many applications a certain turbo model may have the same compressor on a few different applications, but turbine housing is different in all of them. I at lost in figuring out a suitable turbine housing/wheel size for me.
I believe I have hacked compressor side sizing.

Any comments?

 

[mattsooty]

I think that you may be getting hung up on the whole boost thing.

In a gasoline the whole affair is pretty much restricted by how much air you can get into the cylinder, hence a huge requirement for boost however in a diesel the restriction is how much fuel you can get in the cylinder. In a stock CI engine you can *usually* drastically increase the fuelling (and hence torque) with little ill effect except for black smoke and high exhaust temps.

As for 2 stage turbocharging how are you going to map this thing? Honestly speaking you are going to struggle with boost actuators and fuelling control.

If you do have a good means of calibrating your engine then why dont you just get a whopping great turbocharger some good uprated exhaust components and then use the multishot injection potention of a common rail DI setup to ensure that your turbo spools up as soon as possible.

The proper matching of turbocharger & engine is, from what I have seen, very much a black art in On Highway diesel engines - hence the use of VGT/VNT turbos by auto OEMs. For the offhighway guys most of their matching is done over the lug curve with less attention paid to transient response in the partload regions - however I sense that you want/need a bit of both.

MS

 

[MikeHalloran]

If you want your Rabbit Diesel to run harder, it would be _much_ cheaper to just transplant a gasoline engine, even a V8. Tuning multiple turbos is going to be like tuning Weber carburetors; when you're done, if you're ever done, you'll have buckets of exotic, expensive parts that are of no use to anyone. If you really intend to stick with the four, don't forget to first armor the bottom end.

I forgot which is the correct definition for turbo- compounding. The Paxman turbos are not mechanically linked to anything. My friend Wayne spent much of WWII building the speed increaser gearboxes (multistage planetary) that drove turbosuperchargers for B29 engines. The gearboxes were built in much greater numbers than were the engines, because their primary failure mode was complete fragmentation.



Mike Halloran
Pembroke Pines, FL, USA

 

[Fabrico]

This is sounding a little like the F1 saga. When they went out, the turbo systems were capable of making far more power than the engine itself. They were chomping-at-the-bit to mechanically take power from them. Turbo technology is held back by economic and smog requirements. If it weren't for this, they would quickly become turbines and replace the piston engine altogether.