Turbo Boost and FE

[GasSavers_Gollum]

Oh, I was just thinking about something... On top gear, at the begining of the new season Jeremy did a test. He followed a prius around a track following in a BMW M3. The prius was driven all out, and Jeremy just had to keep up in the BMW.

The BMW averaged 2mpg higher than the Prius.

Why so??? The BMW is more in it's range off efficency being pushed that hard. While the prius was going well beyond it's peak torque to squeeze out the most HP possible, the BMW was just reaching peak torque, and just loafing along easily for that engine.

MPG is about so much more than how efficient an engine is.

As I stated earlier in this thread. The only way I could see a turbo having a significant improvement on FE would be if it's actually IN it's efficiency range while at cruising speeds, which doesn't leave much room if you actually put your foot into it. The turbo will reach the end of it's efficiency and overspool badly. Is it possible to really find a balance? It's worth experimenting with.

[dkjones96]

Remember that the range of efficiency is increased when a turbo is applying boost. The torque curve flattens out quite a bit meaning the engine can revv higher and still be reaching good efficiency.

Torque is a direct measure of engine efficiency.

[GasSavers_Gollum]

Yes, but you can increase torque by imrpoving combustion quality, or the amount of air/fuel. By porting a head and changing a cam you can increase torque and HP, but have exactly the same HP per cubic foot of air.

For fuel efficiency you want to make LOTS of power with as little AIR as possible. It's completely different than just looking for more torque or power.

[R.I.D.E.]

Turbo and or supercharged, allows you to use a smaller engine for the same weight vehicle. My project is a 71 karmann Ghia with a 1 liter Yanmar diesel engine. I may add one of the stock turbos from a Nissan 300 twin turbo for better power (the turbo would be free). That way I should have decent performance and exceptional mileage.

regards
gary

[dkjones96]

Yes, that is true, but most efficiency is lost because of the relatively cold chamber walls and cylinder absorbing a lot of heat.

When you are at part throttle you lose A LOT of energy this way and is one of the reasons why engines are more efficient the more throttle you are giving it. The turbocharger or supercharger is giving you more efficiency under boost for the same reason. The percentage of heat lost to the engine itself is less because the ratio of volume to 'heat sink' area is more.

Think of it like your ac in the car. On low, even on the hottest of days you can get cold air, it might not mean much because the ambient temp is so high but it is cold. If you turn the ac up(open the throttle more) the evaporator(engine block) can't absorb as much heat from the air(the burnt fuel in the cylinder) so you have more air(power) even though(because) the temperature difference between the ac inlet(fuel burning in the cylinder) and the vent to your face(spent air/fuel exiting the cylinder) is less than when the ac is on low(part throttle).

[GasSavers_Gollum]

oOo, but here's the clincher... is it that the cylinder walls are cold, or that the chamber is too hot?...

Most engine designs that remove the poppet valve can run upwards of 13:1 compression ratio EASILY due to more even temperatures that don't exhibit severe hotspots.

Those engines see HUGE improvements off efficiency for every molecule of air and fuel put into the engine. Some engines have even seen 16:1 compression on regular pump gas. That's increadible! And that's also far more gain in efficiency than any turbo will give you. If your goal is maximum mileage, you shuold be trying to figure out how to LOWER temps inside the engine, not raise them. This is why water injection can improve mileage. It allows you to keep the chamber colder, and run higher compression, more timing, leaner fuel mixture, of a combination of any of these.

Making an engine hotter or colder on the intake/chamber/or exhaust can be a good or bad thing depending on what's doing it, and what you're doing to take advantage of it.

EDIT: Something else to consider, is that turbos are usually less efficient (take more gas) at a given HP level than natural aspiration when driven under full engine load. Why? The turbo heats up the intake temp, meaning you can't run as advanced of timing, and you have to richen the mixture slightly to keep the cylidner slightly cooler. I'm worried running even mild boost while cruising might consitute similar precautiosn negating any gain.

[GasSavers_Gollum]

Gah, I really dislike these blanket assumptions about SMALLER being inherently BETTER.

I suppose if you had two identical engines but one was 70% the size of the other, it would get better gas mileage, as long as the loads they saw where the same in proportion to the engine size.

BUT, if you take the two engines and put them in the same car going 80mph, where the wind is starting to create quite a load, I bet fuel efficiency will be nearly identical. The smaller engine, though uses less fuel overall, will be needing a larger percent of it's power to keep the car at that speed.

Fact of the matter is that's not so much about engine size as engine technology, hence why I said if we wanted to compare a DOHC V8 from the 60's that it'd be more comparable.

Smaller engines aren't usually MORE efficient than a larger engine built to the same specs. But they're usually in SMALLER vehicles, and due to having less omph people aren't driving as fast or agressive as they would be otherwise.


On my commute this morning I was thinking though, that we could consider the ERG a good counter comparison to a turbo. A turbo is raises efficiency to gain HP, but only under higher load conditions, while and EGR can imrpove efficiency to gain FE, but only under low load conditions. Both alter the temperatures seen by the engine, and guess what? The EGR is more proven as a FE device, and it's COOLING exhaust temps, not raising them...

[dkjones96]

Yeah, smaller isn't always better.

People use 454 Chevrolet engines for irrigation pumps because under the loads they see you can't beat the efficiency(or reliability) of the big block engine.

They could use smaller, high speed pumps and use a smaller, higher revving V6 but it isn't as efficient.

[GasSavers_Gollum]

Exactly, it's about displacement to load ratio technology allows.

But I'd say that most of the amazing technology we see on production cars right now was availible back in the 60's.

The Ford Cammer was making about 93hp per liter, and nearly 80 torque per liter. Not bad for a 7 liter engine. It was also carborated of course, and running mechanically/vacuum controlled ignition.

But it was also designed for leaded gasoline. Running it on todays gasoline the same exact engine would have to have reduced compression, robbing it of power... But if you replaced the carb with a good EFI system that also controlled the ignition, then the compression ratio could be kept where it was from the factory, making the same, if not more power.

Not bad for 60's tech eh? If rebuilt with low friction coatings (availible then, just not on production cars) then efficiency goes up. Add EGR functionality, and cruise efficiency goes up.

I bet if you took an origonal SOHC cammer engine and massaged it a bit you could see modern fuel efficiency out it, when comparing it to like sized engines (7 liters/427cubic inches).

These engines also had variable valve timing... Not electronically controlled, but they had it. Honda is noted for the first use of it on a production car because it was electronically controlled. It was an inherent feature of the engine, not just an add on. Ford designed it for nascar guys, so that once the car's tires and everything else was up to race temps they could squeeze some extra power out of it.

[GasSavers_BEEF]

but if you don't need the load. if an engine is efficient under heavy loads and you are running on flat ground and just driving yourself around then you can do with a smaller less efficient engine.

my car most of the time runs under 40% load. that being said, it would be stupid of me to put a V8 in it. my efficiency under load would go up but only when I had a heavy load.

the example of the water pump is truely apples to oranges. my load varies from the time I crank the car until I cut it off. an irrigation pump keeps the same load (relatively) and same RPM level (relatively) the entire time it is running.

if you could take a truely sub-standard engine (displacement wise) and put a turbo on it, you would get the mileage of the small engine with the help of the boost when needed. if you could gear it to where your running gear kept you at 1500-2000 RPMs at highway speeds, you would see the benefits of the small displacement most of the time. and you could get out of your own way when the time comes by spooling up the turbo.

I said this before in this thread. dodge/chrysler came out with a concept that used a 3cyl turbo engine. it was no where near fast. it didn't make it to production. it was called the slingshot. I think it was in 2004. that is the concept that I would like to see.

*edit* I don't know a lot about diesels but isn't that the concept behind the little turbo diesels that they used to have in the VWs a few years back and the reason they got mid 40s from the factory?