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Turbo/diesel vs. turbo/gasoline


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A friend who is a laung haul trucker called and asked if I could replace a broken injector and turbo on his 2000 Kenworth tractor. I said I'm no diesel wrench but could give it a try. The first thing I noticed is the turbo has no waste gate?? When I got his new replacement turbo from the shop they said it was purpose built. I never could locate any type of BOV either. charge pipe came off the turbo, connected to a huge innercooler, came outa the innercooler to the throttle body via 4 inch pipe. The engine itself looked like a RB style inline 6 cyl. exhaust and turbo on the passenger side and the intake and fuel pump/lines on the drivers side. The injectors are direct port, i.e. directly in the cylinder chamber. I was able to do the job without any issues, realy just a bit of a greasy job.

 

My question is how the turbo was purpose built?? Why no waiste gate? The boost pressure guage always hit 25psi and stayed, while on the throttle, under load. I never did hear any type of blow off gas when shifting like I hear on my Z. There was no oil/block breather catch can either. the block was vented using a 3/4inch hose pointing at the ground. Also the turbo had a nice strong hiss sound, unlike my Z, it sounds like a screaming whissle @ 15psi. I noticed that the turbo pressure came on quick and strong as hell. I dont know if this type of setup is normal for heavy equiptment. My friend did have the powerplant changed when he got the tractor. It is a Caterpillar diesel.

Just looking for more insite and general knowledge at this point. Thanks..

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Diesel truck engines are large displacement low operating rpm engines. They operate in a narrow rpm range and the turbo system is designed to product a high volume at low psi (5 to 8psi) within that narrow rpm range. Typically the operating range is only 500 rpm - from 1,200 t0 1,700 for example. There's no need for waste gates, blow off valves, etc. for an engine that's essentially designed to work at a fixed rpm for hours on end.

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Thanks John. I assumed this but like I said I'm no diesel tech. I was more curious about setting up a turbo in the same way, more purpose built for my perticular setup on the Z. I stopped by the turbo shop and talked to a tech who said that the extra $$ it would take to "match" a turbo to my system would be better spent on things like water/meth spray, better ignition upgrades, having my exhaust system ceramic coated, and getting a turbo blanket...

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A waste-gate is only needed if the turbocharger is undersized...

 

The original Corvair used inlet restriction and outlet restriction to keep the turbo boost level in check very effectively without the complexity of a (then) unreliable mechanical bypass flapper.

 

Take the stock carburettor off a Corvair, put on a 2" SU and you got 21psi of boost and not a bit more.

Keep the stock carburettor on the Corvair, get a rusty exhaust pipe and drop that special 'turbo muffler' and you found your boost gauge pegged and you detonated to death on a cold February morning...feeling that rush of power and wondering what all that racket was out back.

 

Now, put that 4150 Holley on the front end, a straight pipe at the back end, and mate the first generation turbine with the late model 180 Compressor section... WOO HOO! Then about 28psi and more than enough fuel to make that buggy HAUL!

 

But remember, Turbocharging and Hybridizing is new technology... :P

 

The Corvair used a combination of exhaust backpressure and inlet restriction. Most diesels have the same turbocharger response curve, and they aren't really governed like a petrol engine is either. Less fuel directly means less throttle position in a diesel since you aren't moving a throttle plate but a fuel injection plunger control of some sort. When you lift the throttle on a diesel, you inject less gas and have less energy to spin the wheel. But even injecting all the gas you can, the turbo is sized that it will only spin so fast given the heat/flow input (same as the old corvairs)...

 

I worked on constant speed diesels and natural gas engines, and its' very similar to long haul trucks in that load actually controls the turbocharger speed. Our engines had wastegates. But wastegate position was dependent on load on the engine. At 900rpms, and no load, the WG is 100% open. But put load on it and you see it close. Some would be 25% open at 100% load, others with different turbochargers (Elliott ET18 vs Cooper 18 for instance) would be 0% open. One turbo would make 17.5psig at full load, the other 24psig. EGT's would be the same, wastegate positions were different, turbocharger speeds were different. Engine horsepower as gauged by instantaneous KW meters was identical, as was inlet fuel flow conditions (neat what you can measure when nothing is spared in metrology!)

 

Long and short of it, if you have designed the turbo to work without a wastegate you don't need one. For most AUTOMOTIVE applications which work at PARTIAL throttle (watch that trucker drive in the lower gears...) an undersized turbine wheel will result in boost coming on fast and giving power earlier for a more broad useful power band...which goes to what JC said: they don't operate over very much of an RPM range, so they come on when they come on, and they are limited by the size of the turbine wheel and compressor as to what they can do...

 

5am time to go to bed! :blink:

Edited by Tony D
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Thanks for the input fellas. I had a chance to talk to a heavy equiptment repair guy last weekend. He took the time to show me the inner workings of a cat diesel fuel system. Fairly straight forward but them fuel pumps are a thing of art to say the least, plus purging a diesel fuel system is a pain in the @@@@. The pump I got to work with is a gear driven unit with electro solenoid input from the "gas peddal". The useable power with a full 80,000 lb is 900 to 1700 rpm MAX. Also we took out a 30ft box truck with a bypassing turbo (no boost) and man what a slug!! Its amazing just how much power is made from using hot exhaust gasses!! We went by his shop and he showed me some pictures of some industrial sized turbos, holy crap!! I seen a turbo the size of a 1972 vw beatle!!!!!!!

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Caterpillar introduced waste heat recovery where a second turbine wheel is in the exhaust stream dowstream of the turbocharger and it drives a starter gear engaged on the flywheel ring gear (through a fluid coupling) 2-3% power recovery. That's significant on something like a tractor which is just now coming under pollution control. 3% mote power to the ground without additional fuel in the fromt end!

 

I thought it was neat as he'll!

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I worked on Constellations that had 3350PRT engines on them.

 

The PRT means "power recovery turbine" which were 3 units mounted the the engine that were the hot side of a large turbo that were geared to put the power back to the crankshaft.

 

More current stuff is used by volvo and scania.

 

comp_mechturbocompound_Volvo_D12D_500hp_Euro3.jpg

 

Some are looking into involving electricity by mounting a motor/generator into the center section of a turbo and recovering energy as electricity that can be used to charge a hybrid vehicle type battery.

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