Boost

[Howard]

In today's engines, what is the boost bottleneck? Is it material strength, material stiffness, airflow limitations, heat dissipation?

 

[Fenixgoon]

cost is probably the biggest limitation. you could make some pretty fancy castings of high-grade materials for engine blocks (or PM a block..not sure you can machine the whole thing from billet/hand forgings), but the cost is so outrageous that it wouldn't be worth it.

stiffness might be a limitation in the connecting rods, depending on whether they are sized by buckling or not.

fatigue strength under elevated temperature might be the limiting factor for the pistons, along with some criteria for static compressive strength. same thing with valves and springs - high cycle fatigue, possibly with elevated temperature considerations.

just taking guesses based on what little i know about engines (compared to someone who designs them, anyway)

 

[thedarkwolf]

I'd say gas. Diesels can run a butt load of boost already.

 

[jaha2000]

I'd say gas. Diesels can run a butt load of boost already.

This

 

[Bartman39]

In today's engines, what is the boost bottleneck? Is it material strength, material stiffness, airflow limitations, heat dissipation?

This is if your referring to turbo charging...?

Starts with the stock intercooler then the boost tubes and of course your stock turbo is limited so you move on to different pedestals exhaust manifolds & delivery tubes and turbo's of both size and design... Mainly the latter you mentioned (airflow limitations & heat dissipation) are the main issues... Material strength & stiffness come with higher quality performance parts...

 

[Fenixgoon]

I'd say gas. Diesels can run a butt load of boost already.

didnt even think of that just because of.hownthe question was asked

I wonder how beneficial a small refrigeration loop would be to cool the fuel before or after mixing with air.

added weight might not be worth it

 

[JCH13]

In today's engines, what is the boost bottleneck? Is it material strength, material stiffness, airflow limitations, heat dissipation?

The answer is yes. All of these factors can limit a modern engine, engines are too different to generalize.

 

[monkeydelmagico]

didnt even think of that just because of.hownthe question was asked

I wonder how beneficial a small refrigeration loop would be to cool the fuel before or after mixing with air.

added weight might not be worth it

Ethanol FTW. Seen some crazy boost on E85 over the past couple years. Used to be 10's at the strip were good runs.

 

[Howard]

The answer is yes. All of these factors can limit a modern engine, engines are too different to generalize.

Can I get 40 psi on a modern gas engine, tweaked? What if it was designed from scratch, solely to accept massive boost?

 

[jlee]

Aaron at ATS Racing is running 40psi on his MR2 with e85 (828whp). I'm running about 24 on a stock 20yo motor with forged pistons, steel head gasket and ARP studs...not sure what the limitation would be on pump fuel without water/meth injection, though.

 

[KentState]

Tires.

 

[jlee]

Tires.

Touche. :awe:

 

[Gillbot]

money

 

[exdeath]

In practical terms? Fuel octane and detonation resistance.

You can put all the money into the engine you want, but if you are running 91 octane, you're running the same boost as before.

Run anything else depending on availability, and its no longer a daily driver if there is one station in your entire city 50 miles from you that has race gas or E85.

It's really not THAT expensive to build a strong engine. You can easily and cheaply build an engine that will run more boost than any practical fuel will allow.

What really limits how much boost you can dial in is fuel octane. It's not boost that destroys parts, it's detonation. The peak combustion pressure of fuel just before TDC on ignition with or without boost is still the peak pressure. Adding boost doesn't increase the peak pressure unless the fuel detonates prematurely from the rapid rise in air temperature caused by excessive compression from already hot air that was just compressed by an external compressor.

That last point about compound compression gets to the heart of it. How much boost you can run doesn't really mean anything. You can build an engine with 4:1 static compression ratio and 50 psi boost but you aren't going to make any more power than a standard boosted engine, eg: 8.5:1 @ 20 psi.

So the limiting factor is total compression ratio, which ultimately comes back full circle; that is again to say heat capacity and flash point of the fuel under high compression.

As diesels have shown, when you take fuel detonation out of the picture by simply not having fuel in the cylinder, over 100 psi boost is possible in terms of material strength. With current materials we can build an engine that will handle far more boost than any practical fuel can cope with without detonation.

There is also just AFR. Suppose you get enough air compressed in the cylinders that the mass of fuel required to match it is enough to hydrolock the cylinder :awe:

 

[exdeath]

Suppose we just use direct injection for both fuel and a pure oxidizer, we can then eliminate a lot of the wasted compression and heat since 79% of air is non oxygen.

Then you'd definitely need a new cooling mechanism, something involving using an excessively rich mixture to create a cool boundary layer at the cylinder walls like a rocket nozzle.

Bonus points if the injected fuel and oxidizer in your internal combustion piston engine is hypergolic and eliminates the need of an ignition system.

:awe::awe::awe:

Now you have infinite boost, but now you have a new problem. Now the car will completely rust out inside out on a weekly basis running on hydrazine :biggrin:

 

[JCH13]

Can I get 40 psi on a modern gas engine, tweaked? What if it was designed from scratch, solely to accept massive boost?

For appropriate definition of tweaked, sure! The vehicle might be totally unusable or impractical, but anything is possible with enough money.

Odd-ball engines such as VW VR6 that was originally designed for diesel would be a good starting point. Other engines, like the 3SGTE that JLee mentions, are basically race engines in civillian cars and are also good candidates.

From ye goode olden days in F1, BMW had a 1.5L 4cylinder engine that ran nearly 90psi of boost and made 1,500 hp: http://www.bimmerboost.com/showthre...1-motor-photos-pictures-specs-1500-horsepower

Designed from scratch, well, the world is your oyster. You could run 200PSI of boost if you wanted to. You would make tons of sacrifices in other areas though.

 

[LTC8K6]

Ford apparently got the 3.5L V6 to 550 ft lbs of torque using E85 direct injection and gasoline port injection.

 

[Howard]

For appropriate definition of tweaked, sure! The vehicle might be totally unusable or impractical, but anything is possible with enough money.

Odd-ball engines such as VW VR6 that was originally designed for diesel would be a good starting point. Other engines, like the 3SGTE that JLee mentions, are basically race engines in civillian cars and are also good candidates.

From ye goode olden days in F1, BMW had a 1.5L 4cylinder engine that ran nearly 90psi of boost and made 1,500 hp: http://www.bimmerboost.com/showthre...1-motor-photos-pictures-specs-1500-horsepower

Designed from scratch, well, the world is your oyster. You could run 200PSI of boost if you wanted to. You would make tons of sacrifices in other areas though.

I guess at the extreme end, there really is no replacement for displacement, huh?

 

[JCH13]

I guess at the extreme end, there really is no replacement for displacement, huh?

Indeed.

Looking at things like airplane engines are interesting in that regard. You'll have a 380in^3 (6.2L) 4 cylinder engine making a raging 170-180hp. However, it can sit there and output 170hp continuously for hundreds or thousands of hours. Car engines OTOH aren't used at 100% duty cycles and can get away with a lot higher specific power output, especially with boost.

A little off topic though...