Thought this might be useful for someone…
Well, I promised a friend I’d write something this week-end and I figured some of you might have a few questions about turbocharging that I can answer. So the topic for today is turbocharging. I’ll present the basics of what it is and maybe you can ask questions that will lead to more interesting and more advanced topics related to turbocharging. I’ll be writing this as I go, so sue me if you don’t think it’s coherent…
- Rewind back to the naturally aspirated engine.
Of course, before understanding turbocharging and how it can help get more power out of an engine, it’s necessary to understand how a gasoline engine works (diesel is similar).
A gasoline engine works by using the energy produced by the combustion of gasoline. Combustion is the chemical reaction between a combustible (gasoline) and oxygen (which is found in air). Basically, an engine works by filling a cylinder with gas and air (a 14.7:1 weight ratio of air and fuel will burn all the gas and leave no excess oxygen) and igniting it with a spark. The resulting hot and high density gas then expands by pushing the piston and transferring some energy to the piston, which causes the crankshaft to turn. The rest of the energy produced is in the form of heat which is either transferred to the engine block or still present in the evacuated exhaust gases. In fact, only ~30% of the energy from the combustion results in mechanical work. Over 70% is wasted through the exhaust and the radiator (as the coolant transfers the heat from the block). Enter turbocharging, a method to recover some of the heat from the exhaust gases and put it to good use.
As the engine is turning, it acts as a pump, sucking air into the cylinders. The amount of air it can ingest is usually the limiting factor in producing power. Because gasoline is liquid, it can easily be squirted into the cylinder (in a 240sx, one cylinder is 600cc. The amount of fuel required to get a 14.7:1 A/F ratio if the cylinder is full of air is 0.6g, not much). But 600cc of air is the MAXIMUM that can be drawn in there (volumetric efficiency of 100%). A turbocharger will use some of the energy in the exhaust gases to PUMP more air into the cylinders, by doing so at a higher pressure (atmospheric pressure is 1 atmosphere = 1 bar = 14.7 psi).
- How does the turbocharge do it?
The turbocharger is composed of two components:
. the turbine, which is made to convert the energy from the exhaust gases into mechanical energy: the exhaust gases spin the turbine, which is connected to the compressor by a shaft.
. the compressor, which is a standard pump-type compressor that draws air from it’s inlet and expells it through the outlet. If it’s spinning fast enough, the air coming out of the outlet will be pressurized.Because the air from the compressor is pressurized, it will cram more oxygen molecules in the cylinder than normally. When somebody says he’s running 10 psi, the total pressure of the air filling the cylinders is 10 + 14.7 = 25 psi, which represents a 67% increase (25/15-1) compared to an NA engine. Given additional fuel, this can thus produce 67% more power than the NA engine. Of course, the volumetric efficiency of the turbocharged engine might not be the same as the NA engine (it can be higher or lower), so you won’t always get 67% more power when you’re running 10 psi.
- So what do you need?
Instead of dumping the exhaust gases into the exhaust , through the cat and muffler as in an NA car, you need to insert the turbine in there, so you need a new manifold.
You also need to direct the air from the compressor outlet to the throttle body. A thermodynamic side effect of compressing the air is that it will also heat up. Because hot air is not desired (it increases the risk of detonation), it can be worthwhile (read imperative if running medium to high boost) to have an intercooler cool the air before it gets to the TB.
To supply the extra fuel, you can a) get bigger injectors if the stock ones aren’t sufficient or b) raise the fuel pressure so that the stock ones flow more for the same duty cycle. a) typically requires a new ECU and b) can be done with a rising rate fuel pressure regulator for low ot medium boost (<8 psi).
Well, just as the theory of an NA engine is simple, its implementation is nothing but simple, and the same goes for turbocharging: it sounds simple enough, but implementing it is much more complicated and costly.
Does it work? Formula 1 cars were running around 60 psi in the late eighties, producing up to 1500 hp from 1.5l engines (in qualifying form)! That’s a higher hp/liter than ANY type of racing has ever acheived, including top fuel dragster.
Please, ask questions (or ask for details)!
Ken