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Old 06-08-2008, 10:07 AM   #1
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Power vs economy

These are my observations over a lifetime of studying engines.

My general observation is what makes an engine more powerful reduces fuel mileage.

In 1950 (year I was born) Allfa Romeo was the International Grand Prix champion, in the 1.5 liter class. This is the same size as my VX engine, about 93 cubic inches. The Alfa engine produced 390 horsepower. It was actually 2 46.5 cubic inch 4 cylinder engines bolted nose to nose with all accessories gear driven at the meeting point of the two engines, no fan belts were used. Not sure of the RPM but I suspect it was near 10,000. Two stage supercharging, DOHC and I believe 4 valves per cylinder. This represented the state of the art in automotive engineering at the time, much of the development began in Germany by Mercedes and Auto Union before WW2.

The fuel mileage was 2 MPG!

Modern NASCAR restrictor plate engines produce about 650 HP on the tracks that require restrictor plates. At 180 MPH they can get 6 MPG or about 3 times the Alfa grand prix engine of 1950. When you consider the total aerodynamic drag at 180 MPH, 6 MPG is pretty amazing.

We should understand that greater power does not mean greater economy. Greater efficiency can be measured by two objectives.

The first is the most power you can get out of a specific engine design, the second is the highest mileage you can get from the same specific engine design.

Greater power requires increased strength in components becasue the additional stresses involved require increased strength for reliability. Failure to do so means you have lots of power but not for long. Cooling the intake air charge allows increased charge density. Greater weight means greater reciprocating masses which cost more in energy.

Greater fuel mileage requires a completely different design tactic. Lighter pistons, rods, pins, fewer piston rings, lower tension valve springs, less valve lift, lighter valves, smaller intake manifold port diameters, even intentional increases in exhaust gas recirculation, all can contribute to greater fuel economy in the engine itself. Warmer air intake temps allow better atomization and less pumping losses for every power stroke.

Less mass in reciprocating components because you are not trying to achieve maximum power allow reductions in internal strength without sacrificing reliability. Smaller exhaust tube diameters also restrict the passage of air and fuel through the engine and increase efficiency to a point.

The limit to increasing efficiency in modern conventional engines is the point where your sustainable power is not suffecient to maintain a given speed up steep grades for longer distances. Eevn this can be addressed with things like nitrous oxide, supercharging, and other methods of increasing power for severe operational requirements.

I know a lot of pwople here already know this, but it seems like there are also a lot of people who think the old high performance enhancements, which basically allow more air and fuel into the same engine for more power would also increase mileage. This is simply not the case.

That being said there are improvements in engine evolution that have increased efficiency and power. This is where it gets controversial and becomes the source of much heated debate.

Look at the VX design specifics and you get a good idea of the differences between VTEC-C and VTEC. The average fuel mileage of those two different design philosophies represents an almost 100% difference. It would be good for those who are not sure of the difference to study the two designs, which ar basically the same engine to see what Honda did to make such a difference in mileage.

regards
gary
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Old 06-08-2008, 10:31 AM   #2
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Hi Gary, I could not find any info on the VTEC-C engine on google?

I always assumed the more power you got from a given amount of fuel the more efficient the engine was?

I thought it did not matter what size the engine was? it was down to how much mechanical energy it could make or convert from the squish bang process?

I remember reading something along the lines of diesels are more efficient because they have no air restrictions like a throttle plate, but petrol can't do this because of the air fuel ratios? didn't GM make a variable engine head that could change compression ratios?

I think the disspeared it :/
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Old 06-08-2008, 10:43 AM   #3
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Quote:
Originally Posted by R.I.D.E. View Post
Smaller exhaust tube diameters also restrict the passage of air and fuel through the engine and increase efficiency to a point.
Smaller exhaust ports increase velocity and actually decrease the resistance to moving exhaust through the engine at the lower flow rates normally seen in an engine tuned for maximum efficiency. At high flow velocities the cylinders help to scavenge each other and indeed the pressure in a properly designed exhaust can be lower than atmospheric. It's a question of maintaining high momentum of the exhaust gases..

Likewise camshaft profiles, it's not so much the lift that kills low rpm torque and efficiency as it is the overlap between exhaust closing and intake opening events. In fact I think that a good multi angle valve job optimized for high flow rates at low lifts combined with reduced camshaft timing to just compensate for the increased low lift flow would give better low rpm torque and efficiency while making the same overall horsepower.

It took me a long time to realize that you can trade cam timing and lift for increased flow in the ports and around the valves.. Increased flow at low valve lifts is like having a longer duration and lift cam without the negative side effects of uneven idle and poor low rpm torque/efficiency.

Whether you are building an engine for pure power or for pure efficiency the objective is to make it just strong and heavy enough to do the job, anything else adds weight and cuts overall efficiency.

When racing, removing weight is more effective than adding horsepower for purposes of moving the car around the track as rapidly as possible. Colin Chapman's (Lotus) first maxim of sports car design was to "add lightness"..
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Old 06-08-2008, 10:46 AM   #4
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Quote:
Originally Posted by ShadowWorks View Post
I always assumed the more power you got from a given amount of fuel the more efficient the engine was?
Neither simplistic statement is the whole story. More power does not always equal less economy, nor does it always equal more.

You can get power by burning more fuel, or by reducing waste. Pretty much any reduction in waste will give you both more power and more efficiency.

Quote:
I thought it did not matter what size the engine was? it was down to how much mechanical energy it could make or convert from the squish bang process?
Larger engines tend to have larger frictional losses at a given RPM, though significantly less RPM required to produce a given amount of power.

Quote:
I remember reading something along the lines of diesels are more efficient because they have no air restrictions like a throttle plate, but petrol can't do this because of the air fuel ratios? didn't GM make a variable engine head that could change compression ratios?
Diesels are more efficient for quite a few reasons, and that's one of them. There have been various designs for gasoline engines that operate more like diesels in one way or another. GM currently sells a Direct Injection V6 that injects fuel directly into the cylinders, and doesn't necessarily need to use a throttle (though they are using one). I believe they're currently working on a HCCI gas engine, which would use compression instead of spark to ignite the fuel.
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Old 06-08-2008, 11:42 AM   #5
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More power for the same mass (not volume) of fuel means more efficiency as long as its in the range of engine speed that is useable. High mileage cars use engines that develop the best fuel-per horsepower rate at lower engine speeds, while performance engines do just the opposite.

Higher compression means more efficiency in either case as long as you don't have to retard the timing to prevent preignition.

Power is derived from the difference in the pressure created by the expansion of the combustion of the fuel air mixture.

Diesels have almost twice the compression of gasoline engines, but they also have greater pumping losses, even though they have no throttle restriction.

This statement may seem controversial but think about the energy it takes to compress the mixture to over 400 PSI versus 200 PSI for a gasoline engine. Diesel fuel also has more energy per unit of mass. The difference between compression pressure (loss) and combustion pressure (gain) is the most important reason diesels are more efficient. It takes much less energy to create a perfect vacuum (which never happens) than it does to create compression. Remember it's not called suction losses. Pumping losses include all losses related to moving a volume of air through the engine. You can easily see this fact by watching any compressor slow down as the pressure in the tank increases.

Assume you have 10 square inches of piston surface, thats 4000 pounds of pressure to compress. Assume the expansion of the combustion cycle creates a 10 fold increase in pressure as an average. Thats 40,000 pounds of pressure creating useful work.

Mercedes went to throttleless diesels in 1982. Their claim was 7% increase in efficiency due to that single change, otherwise the engines were identical.

Fumesucker is exactly right about exhaust and intake tuning. Smaller for high mileage, larger for high performance. This assumes you are "tuning" both intake and exhaust for best performance at a specific output you wish to achieve. The object is to synchronize the pressure waves in both intake and exhaust to achieve the best rate of flow for a specific operational range.

I want to make it clear. i never said there are no performance improvements that are not efficiency improvements. My point is in most cases and generally speaking modifications that increase available power generally reduce economy.

regards
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Old 06-08-2008, 11:58 AM   #6
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A good example of an improvement that would work in both cases is titanium connecting rods.

Another would be higher pressure fuel injection.

I posted this thread so those forum members who have a somewhat limited understanding of engine dynamics could hopefully learn the difference between performance improvements and economy improvements. That understanding even if basic would answer a lot of questions about "tuner" modifications versus "economy" modifications.

A great example is Warm Air Intake, exactly the opposite of Cold Air Intake. One is good for economy while the other is good for performance.

An intercooler is another example, certainly would give you more power, but for economy the intercooler is actually detrimental.

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Old 06-08-2008, 12:32 PM   #7
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Quote:
Originally Posted by R.I.D.E. View Post
A great example is Warm Air Intake, exactly the opposite of Cold Air Intake. One is good for economy while the other is good for performance.
It was my impression that CAI and WAI each have two improvements, one of which they have in common. The one they have in common is reduced restriction vs intakes designed to be quiet and/or cheap. The one they don't have in common is the temperature.

How about a ram air system? Could it help reduce pumping losses when cruising at highway speeds? I was thinking of doing a common ghetto ram air using a hose opening hiding behind a grille...
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Old 06-08-2008, 01:21 PM   #8
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If you are talking about getting air from in front of the radiator it would be cold air (LOL not when its 97 like it was here today). I doubt it would help your FE.

Most cars now have ram air intakes, longer intake runners. I remember the Chrysler 300's in the sixties. They had two 4 barrel carbs mounted on long ram air intakes. The manifold went from one bank of cylinders past the valve cover on the opposite cylinder head, to the carburetor that was actually located past the opposite valve cover. The slant 6 Mopar engines also had ram air intakes.

Some of the newer engines have long and short ram air manifilds to the same cylinder.

I did disconnect my WAI last week when the ambient temps went past about 85 degrees here. My VX idles so low that with the warmer air the idle was even lower, and when the radiator cooling fan came on the engine would almsot stall before the load sensor kicked the idle speed up.

The lowest idle speed you can tolerate is also a helo in FE.

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Old 06-08-2008, 03:01 PM   #9
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Quote:
Originally Posted by R.I.D.E. View Post

A great example is Warm Air Intake, exactly the opposite of Cold Air Intake. One is good for economy while the other is good for performance.

An intercooler is another example, certainly would give you more power, but for economy the intercooler is actually detrimental.

regards
gary
Gary! what are you saying! this can't be true man?

I thought a larger colder intercooler would mean more FE? not less, how can this be?


I reshaped my exhaust ports not to make them larger as such, but to change the shape of the exhaust jet into a nice long diamond spike, its basically a delaval nozzle shape port now when before it was just an oval mouth that produced a rounder shaped exhaust jet, its just an attempt to get the velocity up, it did help the spool up but I have no clue what happened to the FE on that modification.
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Old 06-08-2008, 05:51 PM   #10
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I you want mileage the intercooler removes heat from your intake charge. Less heat means more air mass, which means more fuel.

Its basically the same reason you get better mileage in the summer than you do in winter. Not the only reason but that is a different topic.

Any turbocharged engine runs better in the winter, but gets better mileage in the summer.

regards
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