High Compression Equal Better mpg?

[Drew1d]

Originally Posted by cfg83

Hello -

Isn't this part of the fuel economy advantage of diesel engines? Isn't there a relationship between their higher-compression and their lean-burn capability?

CarloSW2

The fuel economy "advantage" for diesel is that there is more "stored energy" in a gallon of diesel than a gallon of gasoline.

Ethanol by contrast, has even less "stored energy" than gasoline, but higher octane. That's the reason they mix 10% ethanol with gasoline. It gives you less fuel effeciency (like -3%) but raises the octane rating by a few points.

[retrorocket]

While it is certainly possible to have too much compression for a given fuel.

High compression is a relative term. We have seen compression ratios climb since before WW2 until a slight reduction in the mid 60's with a real fall in the early 70's then a gradual climb back starting in the early eighties and the trend upward has continued to this day.

Fuel octane (unleaded), emissions (oxides of nitrogen) modern engine controls and combustion chamber designs have all influenced compression ratios.

The fight has always been to have as high of compression ratio as possible given the constraints.


In general higher compression yields higher efficiency.

So yes higher compression is good for fuel economy.

More combustion pressure for the same air and fuel input means more mileage is possible.

Most of the time an engine with higher compression ratios have valve timing that is towards power rather than economy.

That is because the market pushes it that way.
Many people are willing to have a premium fuel only engine for power (Honda Civic Si) but almost no one is willing to buy premium fuel to get better mileage.

If Honda was to build a high MPG engine with 11 to 1 compression like the Si engine has then you could get better mileage with that engine than you can with a 9.5 to 1 compression engine. I would say the increase would be enough to completely offset the cost difference in the fuel but since the consumer does not think that way then why should Honda try to push an idea like that.

Actually the Civic GX is an example of a high compression engine for economy because its only fuel is CNG and that has an octane of about 130. I think Honda could have pushed the limits a little more but sometimes it is hard to increase compression beyond what the engine was designed for without running into other problems.

Early automobiles had compression ratios around 4 to 1.
One reason they increased over the decades was because more modern knock resistant fuels came along. The reason for the fuels was demand for more power and better economy.(compression ratio)

RetroRocket

[lovemysan]

I bumped the compression in my 96 nissan pickup from 8.6 to 1(not sure anymore) to around 10 to 1 using pistons. It got the same mileage. Previous best tank before was 27mpg after was 27.1mpg. It always averaged 23-26mpg before and after. It wasn't worth it really. It might have been better had I tuned it. It also had to have premium and it still knocked a good bit. My brothers did not like borrowing it because premium cost more and it didn't get better mileage.

[silverinsight]

Originally Posted by retrorocket

If Honda was to build a high MPG engine with 11 to 1 compression like the Si engine has then you could get better mileage with that engine than you can with a 9.5 to 1 compression engine. I would say the increase would be enough to completely offset the cost difference in the fuel but since the consumer does not think that way then why should Honda try to push an idea like that.
RetroRocket

The compression ratio of the 5 speed Honda Insight is 10.8:1

http://www.insightcentral.net/encyclopedia/enspecs.html

[mrmad]

I think the question "raising compression ratio increases FE?" simplifies the issue. The shape of the combustion chamber, cam timing, intake, exhaust timing, and ignition timing all effect the combustion process. The engine is a system and if all of these are not taken into effect, just raising the compression ratio may have little effect on performance and FE.

[broodlinger]

Originally Posted by cfg83

Isn't this part of the fuel economy advantage of diesel engines?

I think so. Diesel fuel has 10% more BTU's than gasoline (per wikipedia), yet diesel engines get ~50% more mpg. The difference is in the glow plugs, diesel engines glow warm rather than spark, this allows the pistons to compress way more before exploding.