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Old 05-12-2011, 02:55 PM   #1 (permalink)
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Compression, turbos, and efficiency

Reading Christ's thread about a turbo install, I realized I know next to nothing about compression ratios, turbo chargers, and how it affects fuel economy. In this thread, I want to consolodate all of my questions and hopefully begin to understand the physics behind it all.

I have read before that increasing the compression ratio increases fuel efficiency, but what is the reason behind this?

If increasing cylinder pressure increases fuel efficiency, why not direct-inject gasoline into the cylinder at just the right moment and not worry about detonation (like a diesel engine)?

This leads me to the question of why it's important to have a stoichiometric air/fuel ratio in a gasoline car when it doesn't matter with a diesel?

I imagine a turbo increases the effective compression ratio, and this explains the increase in fuel efficiency. Why not just engineer a higher ratio into the design of the engine?

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Old 05-12-2011, 05:05 PM   #2 (permalink)
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The turbo has a dual benefit - it reduces pumping losses, and makes use of excess heat from the combustion cycle, which would otherwise have been wasted as noise from the exhaust.

The compression issue, I don't fully grasp, myself. I know that there is a limit to comp enhancement, where it just takes more energy than it's worth. Diminishing returns and such. Basically, its a mater of heat and ratios.

Heat, in that you have enough heat for total combustion of the mixture AT THE RIGHT TIME.

RATIOS: mechanical advantage, pure and simple. The more leverage produced, the more energy is transfered to the crank shaft. Increasing comp can achieve this, because it changes the ratio of induced volume to compressed volume. However, it takes energy to compress air and fuel, and there is a point of diminishing returns.
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Old 05-12-2011, 05:18 PM   #3 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
. . .
I have read before that increasing the compression ratio increases fuel efficiency, but what is the reason behind this?
This is the most interesting aspect of a college level course on thermodynamics. The 'dirt simple' answer is the expansion ratio is effectively increased as the fuel-air charge at ignition is increased. But the detailed answer is a little longer ... you might check the Wiki or ask Mr. Google. I'd have to reconstruct my college course notes (from 40 years ago!)
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Originally Posted by redpoint5 View Post
. . .
If increasing cylinder pressure increases fuel efficiency, why not direct-inject gasoline into the cylinder at just the right moment and not worry about detonation (like a diesel engine)?
That is the theory of direct injection engines and promises to make a significant improvement in ordinary ICE engine operation. Everyone is working on their versions.
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Originally Posted by redpoint5 View Post
. . .
This leads me to the question of why it's important to have a stoichiometric air/fuel ratio in a gasoline car when it doesn't matter with a diesel?
It has to do with the 3-way, catalytic converter that needs an oscillation between lean and rich. A balance is needed so the oxides and hydrocarbons combine to complete the hydrocarbon combustion and convert the oxides from reactive NO{x} to N{2} and CO{2}.
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. . .
I imagine a turbo increases the effective compression ratio, and this explains the increase in fuel efficiency. Why not just engineer a higher ratio into the design of the engine?
Weight as a turbo-charger lets a physically smaller engine perform at power levels a substantially heavier engine would require. It breathes like a big bore and makes power like a big bore but it is much lighter so there is less rolling drag and inertial losses (aka., when the brakes have to slow or stop the car.)

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Old 05-12-2011, 05:31 PM   #4 (permalink)
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It comes down, to a great part, to the chemistry of the fuel that is being used.

Increasing the compression ratio, which increases the pressure and heat inside the cylinder creates, for lack of a better term, a bigger "bang" that should allow for more complete combustion and less loss in terms of radiated heat.

Gasoline direct injection allows higher compression ratios, but once again, if the fuel quality is such that it does not react completely, you will have pre-ignition/detonation problems. This has been dealt with through higher injection pressures, better atomization and knock sensing ignition systems by and large.

Every fuel has an ideal fuel to air ratio chemically and diesel fuel is no exception.

It's all about control. Diesel engines are designed to withstand high cylinder pressures and abnormal combustion events, along with very lean mixtures. The fuel allows this.
That rattle that comes from a diesel? Detonation / pre ignition.

As long as the fuel / air mix is prepared properly before introduction into the combustion chamber for reaction, and the harder you compress it, the greater and more thorough the release on the power stroke.

Gases burn completely, vapors burn completely, droplets do not. Because most fuel systems take things down to droplet size, whether carburetor or fuel injection, combustion is never complete and hence aftertreatment is needed to deal with emissions.
If the reaction was complete inside the engine, then it would not produce emissions and the fuel economy would go up. The engine would run cooler.

If I missed something, I apologize in advance; there is so much involved.
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Old 05-12-2011, 06:19 PM   #5 (permalink)
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Ok, my brain re engaged; here's some more. When you look at modern engine designs on the gasoline side, you'll hear about high swirl combustion chambers, tumbleports, etc. This is in reference to things designed into cylinder heads to induce motion inside the combustion chamber to help make the fuel / air mixture more reactive, and therefore more complete. In other words, better vaporize everything mechanically so that it reacts more completely. An example of this is the May "Fireball" combustion chamber design that was used on the 5.3L V12 Jaguar engine as installed in the XJS back in the day. This allowed the compression ratio to be raised to 11.5 to 1 on the pump junk we have here, and lowered the fuel consumption quite a bit. Your Acura is another example; when you look at the cylinder head and the piston crown shape, along with the port shape, they all conspire to create a lot of motion in the cylinder, breaking the droplets down even further and making things more homogenous for combustion. This is the reason Hondas run so clean, and can run low octane fuel on a high compression ratio.

The problem with engineering the compression in with the current fuel systems and current fuel chemistries is ( I think ) NOx production as a consequence of higher combustion temps. Now, this is just me, I find it debateable, but who am I. I believe there's a way around everything, just need the time.

Sorry about the segue..........mind drifts. Anyway, the fuel we have is the limiter along with.....oh better stop now before I rant.
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Old 05-12-2011, 07:23 PM   #6 (permalink)
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Fuel atomization, by the way, is the reason you don't polish intake runners after the injectors. Turbulence helps break up the drops. You can polish them up to within an inch or so, though, and it will still help.
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Old 05-12-2011, 07:24 PM   #7 (permalink)
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Quote:
Originally Posted by bwilson4web View Post
Weight as a turbo-charger lets a physically smaller engine perform at power levels a substantially heavier engine would require. It breathes like a big bore and makes power like a big bore but it is much lighter so there is less rolling drag and inertial losses (aka., when the brakes have to slow or stop the car.)
Why don't manufacturers turbo-charge all cars? It seems a sure bet it would more than pay for itself by avoiding the need to build a bigger engine, and in fuel savings.

Quote:
Originally Posted by cleanspeed1 View Post
Gasoline direct injection allows higher compression ratios, but once again, if the fuel quality is such that it does not react completely, you will have pre-ignition/detonation problems. This has been dealt with through higher injection pressures, better atomization and knock sensing ignition systems by and large.

Every fuel has an ideal fuel to air ratio chemically and diesel fuel is no exception.

It's all about control. Diesel engines are designed to withstand high cylinder pressures and abnormal combustion events, along with very lean mixtures. The fuel allows this.
That rattle that comes from a diesel? Detonation / pre ignition.
My point is, it would actually be desirable for fuel to instantly combust if it is directly injected into the cylinder just at the right moment in the engine cycle. It shouldn't then matter what octane the fuel is since we want it to burn right as it gets squirted in. Diesels do this.

Surely gasoline engines can be designed to withstand the increased pressures, heat and such. Diesel engines do this while burning a fuel that has 30% more energy density.
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Old 05-12-2011, 07:26 PM   #8 (permalink)
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Gasoline burns alot faster than diesel, which means much higher cylinder pressure. Technically, the engine would have to be even beefier to withstand the violence of a gasoline explosion.
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Old 05-12-2011, 07:27 PM   #9 (permalink)
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The problem with engineering the compression in with the current fuel systems and current fuel chemistries is ( I think ) NOx production as a consequence of higher combustion temps.
This is addressed by the "pee bottles" (urea injection) that many modern diesels employ. Could this not be used by gasoline engines?
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Old 05-12-2011, 07:37 PM   #10 (permalink)
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Quote:
Originally Posted by Christ View Post
Gasoline burns alot faster than diesel, which means much higher cylinder pressure. Technically, the engine would have to be even beefier to withstand the violence of a gasoline explosion.
I'm not sure a faster burn equates to greater pressure, but perhaps the pressure builds faster (explosion vs rapid burn)? Afterall, diesel has more energy density, so that would imply it generates more pressure, right?

Common-rail diesel engines inject a small amount of fuel just prior to main injection, and this results in a quieter engine. Perhaps something similar can be done with gasoline that would more gradually load the pistons, rods, etc, and reduce the violent forces they encounter.

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