Question about superchargers
Discussion
Not exactly superchargers but any form of forced induction really.
When you fit a 'charger to an engine the compression ratio needs to be lowered to prevent knocking. This is due to the increase in temperature because of the compressor.
If you could somehow reduce the incoming air (post compressor) to ambient temperature (worlds best intercooler?) would you still need to lower the comp ratio?
Although the absolute pressure at TDC would have increased the temperature would not (in theory) be any higher than if the engine was naturally aspirated.
Any thoughts anyone?
Andy
When you fit a 'charger to an engine the compression ratio needs to be lowered to prevent knocking. This is due to the increase in temperature because of the compressor.
If you could somehow reduce the incoming air (post compressor) to ambient temperature (worlds best intercooler?) would you still need to lower the comp ratio?
Although the absolute pressure at TDC would have increased the temperature would not (in theory) be any higher than if the engine was naturally aspirated.
Any thoughts anyone?
Andy
its been a couple of years since i did any did any physics stuff so i could be off here.
you can reduce intake air temperature to below ambient if you use an air-to-aircon intercooler. where the intake charge is cooled by running it through an intercooler running off an airconditioning type compressor. not a common system but ive heard its been used before.
lets say you have the intake charge at ambient. i belive you still have to decrease the compression ratio though, as compared to a naturally asperated engine, although the intake temperature is the same the pressure is not and so at TDC the temperature will be higher.
by compressing a gas you raise its temperature and so if it starts at a higher pressure, by compressing it the same amount its temperature ends up higher.
however if you have an air-to-aircon intercooler, and can cool the intake charge to a below ambient temperature the temperature at TDC will be the same as a naturally asperate car with an intake charge at ambient. that would allow you to use the standard compression ratio.
exactly how much you would have too cool the air, im not sure though.
plus you have the other problems associated with this sort of system. the extream cooling my produce other side effects that would affect engine performance. the compressor would increase paracitical losses from the engine and i would imagine the cost would be prohibitive.
you can reduce intake air temperature to below ambient if you use an air-to-aircon intercooler. where the intake charge is cooled by running it through an intercooler running off an airconditioning type compressor. not a common system but ive heard its been used before.
lets say you have the intake charge at ambient. i belive you still have to decrease the compression ratio though, as compared to a naturally asperated engine, although the intake temperature is the same the pressure is not and so at TDC the temperature will be higher.
by compressing a gas you raise its temperature and so if it starts at a higher pressure, by compressing it the same amount its temperature ends up higher.
however if you have an air-to-aircon intercooler, and can cool the intake charge to a below ambient temperature the temperature at TDC will be the same as a naturally asperate car with an intake charge at ambient. that would allow you to use the standard compression ratio.
exactly how much you would have too cool the air, im not sure though.
plus you have the other problems associated with this sort of system. the extream cooling my produce other side effects that would affect engine performance. the compressor would increase paracitical losses from the engine and i would imagine the cost would be prohibitive.
Contrary to "popular" opinion, you dont necessarily have to lower the cr of an engine to run boost.
The cr will limit the max amount you can run tho.
So a 7.0:1 ration would allow more boost than a 10:1 ratio.
However, off boost, which is where you spend most of your time, the 7.0:1 engine is gonna be, how shall we say?...er a slug.
In the end, it all depends on how much power you want, what fuel youre using, present cr etc etc.
Needless to say, an efficient, well cooled intercooler with high flow capacity and low flow losses will help produce more boost on an engine with a high(ish) cr.
Hope this helps.
The cr will limit the max amount you can run tho.
So a 7.0:1 ration would allow more boost than a 10:1 ratio.
However, off boost, which is where you spend most of your time, the 7.0:1 engine is gonna be, how shall we say?...er a slug.
In the end, it all depends on how much power you want, what fuel youre using, present cr etc etc.
Needless to say, an efficient, well cooled intercooler with high flow capacity and low flow losses will help produce more boost on an engine with a high(ish) cr.
Hope this helps.
chief-0369 said:
by compressing a gas you raise its temperature and so if it starts at a higher pressure, by compressing it the same amount its temperature ends up higher.
Thats not quite right. The teemperature increase is related the difference in pressure, and has nothing to do with the absolute pressure. So, in theory, it should be irrelevant what the pressure is at TDC, the key is the incoming air temp.
If this is ambient temp then you wouldn't in theory need to reduce the compression ratio. I think.
Andy
andytk said: Thats not quite right. The teemperature increase is related the difference in pressure, and has nothing to do with the absolute pressure. So, in theory, it should be irrelevant what the pressure is at TDC, the key is the incoming air temp.
If this is ambient temp then you wouldn't in theory need to reduce the compression ratio. I think.
Andy
The problem is that the incoming air has already been compressed, so it's going to have heated up anyway. Even if you manage to get it back down to ambient, there's going to be a much higher pressure in the combustion chamber at TDC. Here's an example (very much simplified).
Normally aspirated engine. CR 10:1
Air comes into cylinder at 1 bar (I said it was simplified).
Piston reaches TDC. Pressure in combustion chamber is 10 bar.
Forced induction engine running at 2 bar. CR 10:1
Air comes into the cylinder at 2 bar.
Piston reaches TDC. Pressure in combustion chamber is 20 bar.
That's an increase of 18 bar, against 9bar for the NA engine. As you run higher boost, the temperature difference will get even higher. Temperature WILL be higher in the forced induction engine, even if you start by getting the charge temp down to ambient.
When people say that you don't always need to reduce the CR of an engine if you give it forced induction, they're referring to light pressure turbos, where the difference won't be that high. If you're going for a decent bit of boost, you'll need to reduce CR.
James
The way it's done at the design stage is to guesstimate then reduce the CR needed and build a single cylinder engine with the correct head configuration, bore, stroke, rod length. The engine is run on reference fuel (spot on the minimum octane, eg 95 RON exactly) and optimised. Then the CR is increased. Again the engine is optimised. Basically they repeat until the power is maximised but brake specific fuel consumption and emissions are minimised. Once a full engine is built the exercise is repeated to verify that with the full inlet and exhaust system that the CR is right.
The more boost you use, the higher the cylinder pressures, the more risk of detonation, so the more you need to lower the CR or use better fuel to ensure it doesn't happen. Boost gives an effective compression ratio raise, hence the need to control detonation by good combusion chamber design and effective induction and ignition controls.
The more boost you use, the higher the cylinder pressures, the more risk of detonation, so the more you need to lower the CR or use better fuel to ensure it doesn't happen. Boost gives an effective compression ratio raise, hence the need to control detonation by good combusion chamber design and effective induction and ignition controls.
lawrence1 said: Exactly Gavin. And when you cool the air from an inter or aftercooler, the air is more dense, (thicker) so the effective compression ratio will again rise more above that of the static - taa daa more power!!! (but watch the pre-igntion)!
Temperature does not affect the compression ratio. The power increase is simply due to the increase in the amount of fuel and air.
GAvinPearson said: Agreed. A ratio is a ratio. However, the hotter the air is, the less dense it will be. And you will end up with less power. Drive on a really hot day in a modern car where the air is at 35 Celcius, then the next day when it's 20 Celcius and the car will feel much more lively.
mmmn, personally I find modern cars have ECUs to compensate for this, I found older, primitive engines reacted to temp/humidity much more due to the fact a carb has no way of adjusting itself. Another point I noticed from this thread, turbo engines should be particularly adept at making the most of a high octane fuel. My Saab performs exactly the same wether I feed it optimax or cheapo stuff?
Is the gain to be found in a more efficient burn, ie better mpg for a higher octane in this instance?
>> Edited by apache on Thursday 22 May 01:59
I had a Cougar 2.5 and the drop in performance as the ambient air got to 40 celcius was very noticeable, especially compared to the 20 Celcius first thing in the morning.
My wife's Beetle Turbo gives noticeably better mpg and performance on the higher octane fuel we can buy here, basically because the ignition can be advanced all the way through the map and improve output and bsfc at every speed / load condition.
Generaly the more sophisticated the engine management system, the more noticeable the difference will be.
My wife's Beetle Turbo gives noticeably better mpg and performance on the higher octane fuel we can buy here, basically because the ignition can be advanced all the way through the map and improve output and bsfc at every speed / load condition.
Generaly the more sophisticated the engine management system, the more noticeable the difference will be.
apache said:
My Saab performs exactly the same wether I feed it optimax or cheapo stuff?
Is the gain to be found in a more efficient burn, ie better mpg for a higher octane in this instance?
Neither. You will get better performance and economy, unless you use the performance gain to boot it more and lose economy....
If you do not feel any difference then your car does not operate in such a way to take advantage of higher octane. For example an engine management system that advances ignition until it detects knocking (via a knock sensor) will be able to take advantage.
In the SAAB turbo, there is a boost controller known as an APC. It uses a knock detector to retard boost under detonation. If you use higher octane, then the APC doesn't have to come in so often to protect the engine from detonation/knock. Result: better performance.
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