kartweb

Turbo dudes think about this;

Gazing over some some data from one of our new motors I was amazed at some of the exhaust pressures that it produces. OK its not a wankel, its a diesel and a large one at that. 4200 cubic inches. But it made me wonder.

There are three states worth mentioning; Spool up, steady state, spool down. It runs a constant speed (1800 RPM) and when the driver pokes the throttle it loads down the alternator, spools up the turbos, and off she goes. By the way it's the latest from Detroit Diesel Corporation and I would say they definately know what they're doing.

Spool up would see manifold pressures in excess of 180 PSI for half a second or so and drop quickly to about 100 PSI, then fall to about 70 PSI as the turbos spooled up to steady state. Steady state pretty much ran 60-70 PSI. Spool down would drop as low as 30 PSI.

Of course that got me thinking; I wonder what a turbo RX8 produces in exhaust manifold pressure? I doubt anyone has tested it but it would be an interesting tidbit to know.

We had a few of the DDC engineers here this week from Germany and one of them went into some very interesting theories about exhaust manifold pressure on a turbo motor and how it could be used to benefit efficiency through reduction of exhaust thermal losses. I asked him if the same principals applied to a wankel and he was very reassuring it would.

Most NA gas engines see peaks of about 30 PSI. Never really tested a turbo gas engine in the past. Just wonder if anyone else has and what they measured.

Turblown

I've checked this on every Rx7 I've built. Christ I never see anything that high! Infact I've never seen over intake manifold pressure on any car I've built( and yes I don't run 3" exhaust or puny turbine/housings). However this is just with an analog gauge, I haven't used a super sensitive pressure transducer and recorded it. Interesting....

anewconvert

Someone correct me on the physics if I am wrong, but I was under the impression that its impossible for the intake pressure to be higher than the exhaust pressure on a turbocharged engine. Akin to the idea that you cant create energy you cant create more pressure than is driving the compressor wheel or the wheel would stop producing pressure and slow down.


BC

Turblown

Turbines are driven from heat and the expansion of gases, not exhaust pressure. Same way turbine jet engines work.

rotorocks

which push on the turbine blade with forse, which when applied to that blade, and the walls of the manifold is called pressure.

As per wiki: "Pressure is the measure of force applied over a certain area."

staticlag

so according to you, when you heat a gas it doesn't produce pressure?!?

rotorocks

which actually means that, cold gas compressed to 50PSI will apply exactly the same ammount of forse agains the turbine blade, as hot gas at 50 PSI.

...I know, a bit off topic, but just to clarify the concept behind the turbines.

PhillipM

I'd guess that to be pressure before the turbines which sounds about right, takes a lot of pressure to spool a big turbo.

rotarygod

This is not true. It seems strange at first. We must not think of what is going in and out of the engine as only pressure. We must think in terms of total energy. Remember that our output is not only air, it's also heat into air, the coolant, and in our case the oil as well as ambient. Our output is also power to the wheels. If our engine made zero power and emitted zero heat yet spun over, then it would be impossible for intake pressure to be less than exhaust. This too assume that intake and exhaust flow capability are equal. Intakes typically have the ability to flow more as the total area of the pipes is usually greater. This alone has alot to do with it.

I remember back around '97 or so my buddy Marcus was building a tube frame 3rd gen RX-7. The engine he used was a half bridgeport running on Methanol. Using a very large single turbo he ran into a brick wall of power around 20 psi. He would up the boost but power wouldn't go up. Heat would. EGT's were very high. At around 20 psi of intake pressure he was getting 24 psi of exhaust pressure. Since this was a high overlap bridgeported engine that means he had 4 psi of exhaust gas diluting it's way back to the intake side. When he upped the boost pressure, exhaust pressure went up more severely which explained what was going on.

He tried 2 smaller turbos. The wheels are lighter which makes them easier to spin and the total flow area is greater than the single. After figuring out what combination worked he found that his spool rate was faster than with the single turbo. Power was much higher as well. At 18 psi he was topping what the single could do at over 20 psi and it was doing it with faster spool up and lower temperatures. The really neat thing was that he could keep upping the boost and power would keep going up. At 21 psi of intake pressure his exhaust pressure was a little over 19 psi. I know this sounds impossible but remember it's total energy in the system and not pressure that counts. This was the first time I had ever seen this happen. Keep in mind that due to the fact that this was a race motor, he had a much larger freer flowing exhaust and turbine combination than anyone would ever use on a street car. On a street car you just aren't going to see lower exhaust pressures than intake pressures.

Later on after some studying we learned that turbocharged Indy cars actually had lower exhaust pressures than intake pressure. Keep in mind this still isn't free power. It is an increase in efficiency though which feels like free power. The reason it's not free is because it still takes exhaust to spin the turbo. The proof of this is the fact that the exhaust pressure is greater than it would be if the engine were still naturally aspirated. We see an increase in exhaust pressure but an even greater increase in intake pressure. This is an overall increase in system efficiency. It is definitely possible and has been done many times. Most people have just never seen it. Not even those that build turbo systems everyday. Street cars and the optimum setup to accomplish this are two totally different needs.

anewconvert

So long story short you are saying that we arent 'creating energy' because the exhaust pressure isnt the only factor coming into play to spin the turbine. Since heat is also energy you have to account for its seperate effect on the process?


BC