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Old 05-27-2007, 08:13 AM   #1
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my view on engine thermal efficiency

Ok I might as well share the core element of my current idea for all to ponder and give me input.

This was written for someone else and I have pasted it in here.
This concept is what I wanting to put into use in a project car.

In a pure hybrid electric car all of the energy comes from the gasoline engine.
The power requirement at a given moment has been decoupled from the direct load on the engine. There are many ways to do this but the hybrid electric is one that is bearing fruit at the moment. So when the engine runs, it does so with a heavy load therefore the thermal efficiency is higher. So you could say that the engine rather than run at a 15% load 100% of the time that it instead runs with a 100% load 15% of the time and presto the thermal efficiency is much much improved.(slightly simplified version)

The old way of doing this was with a combination of gearing and displacement.
To see the connection I am trying to make lets look at this from a different perspective.
What would be ideal, for thermal efficiency, would be cruising down the road with a very few number of pulses and the engine operating at a cylinder pressure that would be close to the limits of what the fuel would permit.(almost in detonation)
What we see in this situation if we look at it in just the right way is that the engine is making a power stroke 15% of the time but the stroke is being made with a 100% load.

One problem is that when you come up to a hill you will slow down if you do not somehow increase the number of pulses or the strength of the pulses you have per unit time. I personally am not interested in increasing the power within each pulse. (to suggest that there is more power available per pulse is to admit that the engine could be operating at a higher combustion efficiency) That is against the new rules so to speak. It is the lazy way out of the problem. The one that has been used too much. So what is needed is more pulses per unit time.

The hybrid electric has an easier time of getting more pulses per unit time because it just uses more power out of the battery pack and then cycles the gas engine a little more till everything is in equilibrium again.

This comes at a cost however because the generator is not 100% efficient the batteries are not 100% efficient and the electric motor is not 100% efficient.

A straight gas engine would have to add displacement or change gearing to do what I am suggesting.

Now for the paradigm shift.

Really changing gearing is adding displacement.

It is displacement per unit time that is meaningful not static displacement of all the cylinders volumes added together.

Think of it this way.

The 3 cylinder metro can rev the same rpm as the 4 cylinder and both be cruising at the same speed but the 4 cylinder has 33% percent more pulses that are forced to be weaker than the fewer stronger pulses from the 3 cylinder engine.(if total power output is to be equal)

If you simply reduce the gearing so the the number of pulses is the same for both engines you would see that displacement per unit time is now the same and the mileage would also be very close between the two engines.

The important thing here is the true dynamic displacement is dependent upon the number of pulses per unit time and the amount of air going into the engine (manifold pressure, engine vacuum).

See we have had on demand displacement (GM's new term for dropping cylinders) for years and years, just press on the gas and you have more displacement! (dynamic displacement)

So a 302 ford V8 running down the road at 700 rpm would be dynamically displacing the same as a 151 cid 4 cylinder engine running down the road at 1400 rpm. and have the same number of pulses.


The two key components are that the cylinder pressure needs to be high (low vacuum) to have good thermal efficiency and that term displacement as it is normally used is not a very useful concept, in fact it has distorted what is actually going on.

Displacement for a 4 stroke engine is all of the swept volumes added together.

Displacement for a 2 stroke engine is all of the swept volumes added together.

For a 4 stoke engine to displace that volume the crank must turn 720 degrees.

For a 2 stroke engine the crank must turn 360 degrees.

Displacement for a rotary is more misunderstood than the other two.

The rotors have the combustion chamber in it as a matter of fact each rotor has three combustion chambers in it.
So while 12a engine is rated as a 1.1 liter really I think of it as a 3.3 liter with the crank having to rotate 3 times 1080 degrees.

The point here is displacement is an artificial concept.
While dynamic displacement of displacement per unit time is a very useful and real concept.

I feel that low vacuum and fewer stronger pulses per mile is the key to engine efficiency.


Here is a video of one of my max fuel economy intake setups.

http://video.tinypic.com/player.php?v=16kp2s1

Called the QUADQUAD

It is not as far fetched as it looks at first.
That is because there is no plenum.
It is a true IR setup. So the cylinder pulse to venturi area ratio is the same as stock. I will be happy to explain more on this but really I just put it in here for fun. It was designed for a street strip application and fuel mileage was not really a design objective.

retrorocket
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Old 05-27-2007, 11:25 AM   #2
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Not set up for fuel economy??!! YA THINK. Cool design and alot of good insights. You have alot of good points. We have seen Z06 vettes with 505hp pulling down 28+ mpg on trips, aero, weight, built in belly pan, and 1200rpm at 70 makes the difference. AFM on the new Buick Lacrosse is seeing 28.3mpg at 74mph. Can't get an eng. to slow down like I do to see if it will break 30. As you say we can have active systems, use your brain and foot.
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Old 05-28-2007, 07:32 PM   #3
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I found this link close to 3 times the stock mpg

A very rare image of just what Charly's 185 Yamaha looked like before the streamlined fairing* was installed. Charly says:
"Our chopped and lowered tall geared bare bike got about 200 mpg - mileage leaped to 300 mpg when the fairing was installed"

http://www.craigvetter.com/pages/470...20winners.html


I think the stock bike got about 70 mpg at the most

so they got it up to 200 with about 100% load on the engine and super tunning.

What do you think?

can a large improvment like that be made on a car? or is a bike just that far off from the factory?

470 mpg

http://www.craigvetter.com/pages/470...PG%20Main.html
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Old 05-29-2007, 03:56 PM   #4
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Yes, large improvements like that on cars are relatively easy following the same pattern, or even just changing gearing. I'd guess that the optimum is ~100mpg@50mph for the average car, or ~50mpg@50mph for the average pickup. Of course, this isn't exactly practical for most in terms of top gear acceleration, so a range of ~50-70mpg and ~30-45mpg is probably a decent combo of practicality and efficiency, w/o going all Basjoos on the vehicle.
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Old 05-29-2007, 06:34 PM   #5
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Yes I think so too. I'd love to see someone with the skills in body work do that to my Geo. I'm up for the experiment.
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Old 01-28-2008, 07:06 AM   #6
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I've recently made two trips from Albuquerque to LA. One in my car and another in a 2006 Grand Marquis for business. My 2.0 tracker got an average 24.5mpg with an average speed of about 80. The GM got average 28.3 at 85. These were an out there and back, leaving Saturday and back Sunday, trips taken one week apart(1745 miles round trip).

My car turns 3950 rpm to keep 80, and for numbers sake, don't count the fact that around Flagstaff, AZ at 7000ft I went a total of almost 50 miles stuck out of overdrive because the little engine couldn't keep that speed without being pretty much wide open in 3rd(5200 rpm at 80) for that stretch. That calculates out to 355,588 pulses per mile(true for any speed in overdrive with a locked-up torque converter.

The grand marquis turned 1900 at 85 and didn't need to get out of that in AZ but a couple times. That car runs 321,806 pulses per mile. Double the cylinders and a little more displacement and weighs about 1000 more than the tracker does. It's a boat.

The Tracker not only uses more pulses per mile but the pulses it does use are harder. Granted, the GM had an unfair advantage in this one because it makes peak torque very low and is close to where it was running the whole time. The tracker makes peak torque at 3000 rpm and the engine was revved out past that because of gearing to start with.

Nevertheless, just making an engine smaller and revving higher and using stronger pulses doesn't always work. The GM in this example was hardly ever over 20% throttle. The other car was almost always above 50%. The tracker used 9.5% more pulses but got 13% worse mileage and that's comparing average of 80 vs 85. And as you slow down the span stays about the same or gets worse. At around 60 the GM was reading about 32-34mpg and the tracker never read over 26mpg and anything over 55.
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Old 01-28-2008, 07:54 AM   #7
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The real reason that fuel economy sucks today is that engines are simply too large. Cruising at 60 mph may take 25 horsepower. But even the wimpiest cars have engines capable of 150 HP. Today's engines run at 20% throttle most of the time! And part throttle operation of a gasoline engine really hurts efficiency.

If a carmaker wants to post high FE numbers without expensive techologies like diesel or hybrids, all that needs to be done is reduce the number of cylinders. A single cylinder or twin cylinder car making about 40 HP at 3500 rpm would get Prius-like FE numbers.

40 HP not enough? Remember, the early Beetles came with a 37 HP engine.
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Old 01-28-2008, 09:22 AM   #8
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Yeah, my vw bus had 62 hp, and it was fine in town, but you had to be patient up in the mountains. It weighed over 3,000lbs and had the aerodynamics of a cardboard box.
I am guessing the vx makes about 25-30 hp in lean burn and that is fine for 70% of situations. In a car of this size it is nice to have more power than that available, but the 92 that it has is more than necessary.
Like everybody has mentioned everything has to be considered. A smaller engine making fewer pulses, narrow LRR tires, aerodynamics, light weight, tall gearing, low rpm cam. all of these things need to be included for all out efficiency.
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Old 01-28-2008, 11:41 AM   #9
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Quote:
Originally Posted by dkjones96 View Post
Nevertheless, just making an engine smaller and revving higher and using stronger pulses doesn't always work. The GM in this example was hardly ever over 20% throttle. The other car was almost always above 50%. The tracker used 9.5% more pulses but got 13% worse mileage and that's comparing average of 80 vs 85. And as you slow down the span stays about the same or gets worse. At around 60 the GM was reading about 32-34mpg and the tracker never read over 26mpg and anything over 55.
This was what I was thinking too.

Just because you use more pulses, it doesn't make it efficient given that everything else is the same.

An engine has an RPM where it's efficiency is at it's greatest. Beyond that, your efficiency goes down, but you continue to build HP because you're packing more "hits" in a period of time. As an example, a sporting 600cc motorcycle will make peak HP at the upper end of it's rev range around 12k to 14k RPM's, but it's peak efficiency is probably half that number.

Just some thoughts.


Yeah, it needs to be a work motor. Something that has a specific RPM for work. That can probably be figured for specific applications. Lawn mower, construction equipment, etc; the engine might run at a specific range of RPM's and that power is input into a unit such as a hydro or something.
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Old 01-28-2008, 02:07 PM   #10
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Originally Posted by SD26 View Post

An engine has an RPM where it's efficiency is at it's greatest.
This is normally the point of peak torque which translates to BMEP numbers.

BSFC maps for most engines is a mirror image of the torque curve BUT this applies to the engine on a test dyno only...fitting the thing into a vehicle means a lot of other factors come into the mix.

Pete.
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