Fuel milage and compression

[Anonymous]

Whats the corolation between compression and fuel efficiency. In the 80's the 200's compression was lowered to provide better fuel economy. Why does lowering compression increase economy. I would have thought it would be the other way around. With a high compression you get more power per stroke and a better burn. Can someone please clarify the reasons for this?
Thanks
Paul

 

[Anonymous]

Take a tire and air it down to 5psi and push on it with you foot, does it squish alot easier than with 30psi in the tire? Yes. Works pretty much the same way with compression. The higher the compression(air pressure) the harder the piston/rod/crank have to push against it to compress it. More work = worse mileage. You can get more power out of higher compression but at the cost of higher octane fuel and such since more pressure = more bang to a certian extent.

 

[SuperMag]

Why does lowering compression increase economy. I would have thought it would be the other way around. With a high compression you get more power per stroke and a better burn.

Higher compression requires a richer mix to run the same amount of timing advance that a lower compression engine can (to a point). In other words, a low compression engine can be run a lot leaner.

 

[Anonymous]

There is a combination of parts that can increase mileage and power at the same time. By increaseing the compression ratio your talking about the static ratio and not the dynamic ratio. You can have 2 motors producing the same cranking compression of 165 psi. One can have a static ratio of 9.5 to one but be running a larger cam to where the intake valve closes later after BDC compaired to the 8 to one motor. The 9.5 motor will have a better efficency than the 8 motor because of the larger cam and better airflow numbers and will burn less fuel per hp as a result. It is possible to combind the right parts with higher compression ratio and increase your fuel mileage.
Don't forget back in the early 70's the cam technology wasn't like it is today. The grade of fuel had dropped so much that the manufactures had to lower the ratios and run smaller cams. Aluminum headed motors haven't came into the own yet so the ratio's had to be lowered. Motors today are running around 10 to one ratios and higher and still putting down 20+ mileage for V8s and near 35 for 6s.

I hope some of this makes since.

 

[CZLN6]

Howdy Paul & All:

It is my belief that the factories reduce CR is an effort to improve emissions as imposed by the EPA. This was as a direct result of prohibiting lead additives to increase octane ratings of gasoline. Prior to '72 factories relied on the addition of lead to control knock and valve wear rather than increase technology. After '72 they relied on external band-aides like air pumps and catylitic converters to meet EPA standards. If you follow Mark P. thread in the Hardcore forum, you'll get the ideal.

You may have noticed that todays new cars are back up in the 10:1 CR range on 91 - 93 octane no-lead gas. How did they do that? Technology! Also note that in 1965 premium gas was at 100 octane rating and regular was at 95 -97.

It is also my belief that a properly tuned engine can achieve better mileage at 9:1 than at 8:1. Please place an emphsis on "PROPERLY TUNED".

That's my two cents, for what it's worth.

Adios, David

 

[Anonymous]

Lowering compression does not necessarily increase fuel economy. In the 70's, two things happened simultaneously that resulted in the change in compression ratio and fuel economy.

I think the primary reason that the compression ratios were reduced was due to a chief environmental concern of the time. High compression engines required high octane fuel. A chief anti-knock additive was a lead additive. Starting in the 60's I seem to remember lead being removed from paints etc in an effort to reduce our lead exposure which has a cumulative effect on the body. Lead was being reduced in the fuels, and the current fuel technology did not have alternatives to replace the lead additives. So, the high compression engine was being killed. This was just a part of the overall push to reduce automobile related smog emissions in the cities.

The other push that happened to nearly coincide with the decrease in compression ratios was the oil-embargo. The embargo accelerated a push for fuel economy. But by the time the of the oil embargo, the push for reduced emissions was already killing off most high compression engines. Fuel conservation was championed, and a nice side benefit was the fact that better fuel economy yields less pollution. Nobody "needed" a 400+ HP big block car when you could not get the fuel for it anyway.

At the time, hi compression engines typically had poor idle emissions due to valve timing that was optimized at higher rpm ranges. So decreasing the compression ratio could reduce idle emissions which are a significant portion of the total emissions. The other obvious way to reduce emissions was to increase fuel economy. Limiting the horsepower that the engine could develop was a way to meet emission and fuel economy standards.

As mentioned, high compression does not necessarily imply lower fuel economy. And as Ol Boy pointed out, the technology has changed where we can run variable valve timing etc to increase performance and reduce emissions. .
Doug

 

[Anonymous]

Sorry, but Im still confused. I would have thought that encreasing the energy produced with each stroke would cause more power to be produced. More fuel wouldn't necesarily have to be used just compressing the existing mixture more would produce more power. If more power was produced then the driver wouldn't have to press as hard on the pedal and the because the same power was being produced at a lower rpm. I must be missing it cause im still confused. anyone know of a good article on the subject?
PAul

 

[Anonymous]

Eighty-some years ago, English engineer Harry Ricardo invented the squishband combustion chamber which allowed him to raise compression ratios considerably without running into detonation (using the same fuel), resulting in a big boost in torque AND fuel economy.

Thirty-some years ago, automakers, who had been using squish-type heads ever since Ricardo, observed that the squishband had the unfortunate side-effect of leaving some unburned hydrocarbons in the exhaust. Emmisions being considered more important than power and economy in those days, they greatly reduced the squish area, going to open-chamber, low-compression heads to prevent detonation.

 

[Anonymous]

Pacman.... Don't forget that there's a throtle valve that will reduce the compression ratio. A motor only makes use of the full potenial compression ratio at WOT (wide open throtle) Other then that at idle it maybe running with about 3 to 1 and crusing down the road with about 6 to 1. That hole vaccume thing at different engine speeds and loads.

Diesels run by increasing the amount of fuel added to the combustion chamber at the proper times. Every compression stroke is going to be at the full potenail compression ratio. Now we have even put turbos on top of diesels to increase the fuel economy and power output. 15 lbs of boost on top of a motor already running 19 to 1 compression ratio. With the idea that increased compression ratio reduces fuel mileage then a diesel should be around 3 mpg and not 20 mpg like they are.

I bet the diesel part is realy going to mess you up

 

[Lazy JW]

This is the first time I've ever heard it suggested that the automakers reduced compression in order to increase mileage. I was an avid motorhead during the 70's when all of this happened, read all the books and magazines that were then available, and I must have missed that part. I say nonsense. They reduced compression to reduce emmissions. Period. The fuel quality was down, increasing federal regs, and engine controls weren't as sophisticated as we take for granted today. I say that without the federal regs that we wouldn't have the fine engines that we have today. They forced us into building cleaner, more powerful engines. There truly is a diminishing return on higher compression ratios, but I just don't buy this theory at all.
Joe

 

[MustangSix]

Higher compression translates into higher efficiency (up to a point). Lower compression yeilds less power and lower fuel economy.

The compression ratios were lowered in response to a Federal requirement to reduce NOx emissions, a byproduct of higher combustion temperatures. Lower CR and EGR reduced NOx by lowering combustion temps at the expense of both power and efficiency.

Anyone who has ever owned an emission-strangled car from the 70's remembers the dismal fuel mileage these engines had.

These days NOx is controlled by careful management of fuel, timing, catalysts, EGR, and air injection......all managed by a pretty sophisticated ECU which calculates speed, load, gear, and a multipl;e of other things. The end result is today's engines are far more efficient and cleaner than anything produced in the 60's or 70's.

 

[goinbroke2]

CRAP! Ya beat me to it mustang six!
Your dead on, higher comp is more efficient but causes NOX, can't have too much nox now can we? Lol!

 

[66 Fastback]

Now that I think about it, lead would not have been the reason that compression ratios were reduced in the early 70's. My 73 Comet still ran on leaded fuel. I would have to agree that it was the NOX that precipitated the reduction in compression ratio. Lower compression increased the quench area, cooling and slowing down the combustion process.
Doug

 

[xctasy]

Now that I think about it, lead would not have been the reason that compression ratios were reduced in the early 70's. My 73 Comet still ran on leaded fuel. I would have to agree that it was the NOX that precipitated the reduction in compression ratio. Lower compression increased the quench area, cooling and slowing down the combustion process.
Doug

MustangSix, goingbroke2, right on!

The reason octane rating was reduced was that it results in the most economical use of crude oil reserves from the refinery. Leaded and unleaded Hi octane uses much more crude than low octane. MON/RON ratings below 90 result in the need for lower compression, and less specific fuel economy. Going up octane always allows better fuel consumption. The petrochemical companies do a simple fuel consumption verses crude oil, and came up with a 91 RON/MON rating as the optimum use of the valuable crude.

Since then, electronics and the May/ High swirl, and high mixture motion heads form Chevrolet etc have made the case for fuel companies rasing the octane ratio above 91 again. Type R Hondas and other imports are designed for 100 octane in Japan, and get better power, economy, and no real emissions increase.

 

[Anonymous]

Diesel fuels make better use of crude oil, with fewer refinery steps and equipment, than gasolines. So, why is diesel at the pump costing as much as 91-92 octane gasoline, these days????

 

[Lazy JW]

Another example is my beloved air-cooled VW engine, probably the most fragile, compression sensitive automotive engine ever to be widely used. Those things had 7.5:1 compression stock in the early 1970's and required 93 octane fuel. When the octane rating for regular unleaded was reduced to 87 the poor VW's started misteriously melting down. The late Gene Berg did extensive research and testing and detirmined that the available fuel simply won't support more than 6.6:1 compression and still be reliable. Guess what? Fuel mileage suffers as a result of lowering compresion. I have had success at running full stock compression in my Bug because I treat it very gently and shift down on the hills, NEVER lugging it. But then, a Geo metro will outrun it and get better mileage to boot, along with lower emissions. But I doubt that many Geo's will be getting restored 30 years from now :roll:
Joe

 

[SuperMag]

Higher compression translates into higher efficiency (up to a point). Lower compression yeilds less power and lower fuel economy...

OK.... lemme ask you this (one of those "all else being equal questions"):

Emissions aside, two engines with 80s era management systems (read: carburetors) which run on pump gas each have an output of 100 HP. Engine 'A' has a compression ratio of 8:1. Engine 'B' has a compression ratio of 10:1. Which engine gets better mileage?

 

[Anonymous]

uh.... easy "B motor" will get better mileage. The 10:1 motor will make the 100 hp at a lower rpm then the 8:1 motor. It will also have more turque accross the rpm range. It will take less throtle to get the vehicle rolling and use less fuel. It all comes down to the amount of fuel(lbs) to make 1hp for 1 hour. I think it's called BSDF...? most street going motors require .45 to .55 lbs of fuel to sustain 1hp over an one hour periord. The NASCAR Cup guys are getting something like .2 to .25 for there motors. Alot more efficent at using the fuel to its fulllest.

 

[MustangSix]

All else being equal and fuel octane taken into account, in your example the high compression engine will make as much as 10% more power and use less fuel per hp.

 

[SuperMag]

Interesting... When I lived in Vermont (sea-level), I tuned my truck (1986 F-150 with a non-feedback carb circa 1971) to run as economically as possible- as lean as possible with as much timing as it would stand on 87 0ctane gas. I kept a 1000 mile running tab on mileage, and at the end I got an average of 14.7 MPG, with a one tank-full high of 16.5 MPG.

Since moving here, (4000+ feet above sea level), I re-tuned it for the local conditions, which because of the altitude, involved a leaner mixture, and more timing advance. In very similar driving conditions (hilly, two lane roads with no lights or stops to speak of) I now get 16.4 MPG, occasionally getting as much as 18.0 MPG. On 85 Octane yet...

I assumed that since I'd done nothing to the truck, the increased mileage came from the altitude (less dense air = lower effective compression ratio) and the increased timing and leaner mixture that that allowed...

Thoughts?