General turbo theory questions

fordconvert

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Lots of questions I have never really found the answer to. Im pretty well sold on turbos for diesels and I can see the logic for gaining acceleration performance on a smaller lighter rig. It seems the ideal setup for a gas engine is to size the engine so that it can cruise without boost. This gives you the best efficiency right? You then add the turbo to get you the better acceleration that you would normally not get with that engine you picked based on its cruising power. This gives you very good power for accelerating but does not hurt very much at cruise so its a pretty good compromise because the overall weight and possible efficiency savings of having a much larger engine it would take to get the same acceleration without boost. Is that pretty much the basic concept? Now from what I can see the problem comes if the engine can not cruise without boost, its sort of like being in the secondaries of a 4bbl all the time, takes a lot of gas and is a little more stressful on all the components involved? This could be because of the terrain where you live or the loads you are carrying.
 
You're basically correct.
At cruise you'll never have positive manifold pressure (boost). At least you shouldn't. I'm not 100% sure on diesels, but I'm pretty sure it would be the same. The only thing I can think of that would have positive manifold pressure all of the time would be a turbocharged generator or water pump under a constant load.
So with that we get the real answer to your question. When an engine is under load, and it's turbo'd, it'll have positive manifold pressure.
The reason for that is two fold:
1) More fuel is burned so there's more exhaust to turn the turbine.
2) The throttle blades are open. So the intake restriction is reduced thereby allowing the compressor to spin freely. (or at least at a reduced restriction than cruise)

Later,
Will
 
I'm hesitant to go against will on this but I don't thin I agree. From a quick search, wikopedia say:
The objective of a turbocharger is the same as a supercharger; to improve the engine's volumetric efficiency by solving one of its cardinal limitations. A naturally aspirated automobile engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder through the intake valves. The pressure in the atmosphere is no more than 1 atm (approximately 14.7 psi), so there ultimately will be a limit to the pressure difference across the intake valves and thus the amount of airflow entering the combustion chamber. Since the turbocharger increases the pressure at the point where air is entering the cylinder, a greater mass of air (oxygen) will be forced in as the inlet manifold pressure increases. The additional air flow makes it possible to maintain the combustion chamber pressure and fuel/air load even at high engine revolution speeds, increasing the power and torque output of the engine.

So why wouldn't u want the boost in volumetric efficiency at cursing?

And when cruising you are under a load just not as big as WOT so you will have positive manifold pressure or boost.
 
You don't need the extra boost at cruise. It only takes 20 hp or so to keep a car running at 55 mph, once it gets there. That's not enough of a load to generate boost, just enough to keep the turbine spinning.

I can watch my boost gauge and see that boost goes down to zero when cruising at anything below 75 (in my particular car). Above that, it starts making boost because of the aero drag, and mileage plummets drastically.

Also, volumetric efficiency doesn't necessarily equal gas mileage in this concept. When you make boost, you have to inject more fuel or bad things happen. This is why superchargers get worse mileage: at cruise, they are still making boost because they are tied to engine RPM. The turbo isn't, and can just freewheel until needed.
 
On a typical EFI turbo gas engine is the throttle before or after the turbo?

I am fairly certain that running the turbo hard all the time (at least on a gas engine) is about as inefficient as you can get plus I assume its hard on the whole system. I know of supercharged piston airplanes but are there turbo pistons also?

No throttle on a diesel so lots more air flow (in and out) even at low rpms. The beauty of a diesel is its all about fuel. You want more power and or to make the engine spin faster you just put more fuel in. You want less speed you put in less fuel. You dont have to worry about running lean.
 
Throttle plate is always after the turbo on an EFI system. When the throttle closes suddenly there can be issues, that's what a recirc/blow off valve is for.

There are a lot of turbocharged airplane engines out there, Lycoming and the usual suspects still make them. The P-47 and F4U were both turbocharged.
 
The turbo should only kick into boost when the engine is under load.

Cruising down the highway on cruise control in my 93 Capri XR2 turbo my engine never went into boost just cruising.

Ergo, just south of Louisville Ky where the hills start getting steep as soon as the engine was put under load to keep the engine speed up the turbocharger kicked in and the car accelerated effortlessly up the hill.

You are also not supposed to run the turbo for extended periods of time or you can damage the turbo from overheating. IIRC the main problem is the center section with the bearings gets too hot. In my Capri there was a overboost warning and a turbo overheat warning.

This was a big deal on my older turbo cars. Not sure if it still applies to modern turbos with dry film lubricants, water cooled center sections, and ceramics.
 
:D Hi Wallaka.Almost all of the WW2 aircraft were turbocharged.A little piece of trivia,the turbo on the P47 was BEHIND the cockpit.
Leo
 
Ok got to weigh in here.
Almost NO WW2 aircraft were turbocharged, the technology of turbine materials wasn't well known at that point so it wasnt reliable enough at that stage. Plenty were supercharged, (ROLLSROYCE MERLIN,DB v12, bristol etc) but only a couple were turbocharged (Wright Cyclone, Allison 1710) and centrifugally then.
The statement above about VE was correct, boosting the pressure on the intake is about VE only.
Dont compare a Diesel engine to a petrol one, they are a completely different animal.
A Diesel is a compression ignition engine where the combustion is stratified, thus there is no lean mixture in the sense of the petrol engine, the small ball of fuel right at the injector inside the cylinder is the right mixture to burn, the fire once lit is added to as the combustion continues, all of the fuel isnt in the cylinder when combustion starts, adding more fuel just makes the fire bigger.More fuel equals more heat which equals mor pressure equals mor torque. The Diesel operates always with "excess air", that is ,its always got more air than it needs to support complete combustion, even when not blown, this is part of the reason why the exhaust temperature is lower and a NA engine never has the power of a similar sized petrol engine.
Adding a correctly matched turbo to a Diesel usually improves its thermal effiecncy as some of the waste heat of the exhaust is recovered to improve combustion.
NB a "gas" engine to me runs on a gaseous fuel like Methane or propane.
Most Diesels are not throttled in any way, a lot run VERY high manifold pressures eg up to 500kpa. They also do not suffer from combustion knock, so as long as the engine can stand the thremal and pressure loads you can keep increasing the MAP. Some C15 caterpillars have 300kpa boost. They get good life and reliability in average service, down here in Australia they do blow up from time to time, we generaly haul much heavier loads than the USA and in very high temperatures. 42-48c.
Back to petrol engines, generally the turbo or blower is added to increase VE, it generally does not improve thermal efficeincy in any way, usually makes it worse. So thats why there arent many turbo petrol engines in use, SAAB tried smaller turbo engines to try and improve fuel effiency but didnt get a lot out of the excercise. The whole trouble is because the petrol engine is a homogeneous charge, so they cannot be run any leaner than the lower combustable limit, the fire simply goes out.
Overall if your attempting to improve fuel effiency, get a Diesel, a small direct injected turbo Diesel like a Duetz.
My two pence worth
A7M
 
:oops: Hi Aussie7mains.You are correct.Two stage superchargers were used on quite a few
of the AC.Thanks for the reminder.Merry Christmas to all our friends from Australia and a Happy New Year.
Leo
 
aussie7mains":3g0jaxqn said:
So thats why there arent many turbo petrol engines in use, SAAB tried smaller turbo engines to try and improve fuel effiency but didnt get a lot out of the excercise. The whole trouble is because the petrol engine is a homogeneous charge, so they cannot be run any leaner than the lower combustable limit, the fire simply goes out.
Overall if your attempting to improve fuel effiency, get a Diesel, a small direct injected turbo Diesel like a Duetz.
My two pence worth
A7M

I would have to disagree with this point. Case and point I look at my two volvos. One is a 2.3L 5 cylinder that is turbo charged (and petrol) to 15 psi. That 2.3L makes 250HP. A similar ford 4.6L of the same year made 250HP (or close to it) but got worse mileage (in a mustang). Small turbo charged gas engines are quite solid, just not popular especially in countries where the mantra has always been "More displacement". Small turbo charged engines are the key to increasing economy standards and still giving people the power they want out of a car. When heading down the highway, my volvo is not operating under boost. Punch the gas and shortly there after I get up to 15 psi and the car goes screaming down the road. But I still get to enjoy 30 MPG on the highway.

Also, my T-5 (the 2.3L variant) gets the same mileage as my GLT (with only goes up to 5psi at 190HP) which gets the same as the NA (155HP) version. All use the same basic block and manifold. the GLT and NA used a 2.4L block that was just a slightly more bored 2.3L block. For the high pressure turbo block they wanted more sidewall thickness on the block, but the block is the same design.

Turbos don't necessarily gain efficiency, they just gain on demand power while still giving efficiency when the power isn't needed. Turbos are more a compromise of demands. Power and Efficiency. Longevity of a good system is good as well. My T5 is on the original turbo at 210k miles.
 
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