a technical discussion, engine efficiency Options

[r_towle]

so, I have been thinking (not always a good thing)
If an engine is basically an air pump, and the objective is to make it as close to 100 percent efficient, why is the exhaust valve so much smaller than the intake valve?

Forget emmisions, forget fuel delivery, just want to understand why this is so...it is so on most engines...

Rich

[michel richard]

Cause on the exaust stroke there's a hard piston pushing gases out, while on the intake stroke there's only soft atmospheric pressure pushing the mixture in. I suppose the exaust valve is made smaller because larger is not needed, and would there fore be a waste. Plus there's a ton of other concerns such as gas speed etc . . . that I don't fully understand.

FWIW, my .02

Michel Richard

[r_towle]

I thought that there is a vacuum sucking the mixture in...

To obtain maximum airpump efficiency, we are taught to open up the exhaust (that is what I am told is the real restriction on these motors)

So...why not design it from the start to be somewhat closer in size.....

Rich

[Jake Raby]

......... Because that about 20% of the air fuel mixture is burned completely and the density of a burned mixture is MUCH less than one thats atomized with fuel...

I have built some awesome Turbo engines with the same size exhaust valves as intake, with a really modified cam to promote it... BUT don't ask them to run very well N/A

[r_towle]

thanx for that reply Jake...as always you get to the point.

Rich

[Jake Raby]

Rich,
The summary I made was very general... With some things like this I fully understand them, but lack the ability to put them into words.

Remember that bigger valves are heavier and actually "Get in the way" at certain valve lifts, especially @ full lift.... From what we have seen big huge valves are a certain way to kil the TIV powerband and power output.

It's ALL in the combo! Buy a flowbench and dyno and you'll see that daily.

[messix]

the exhaust is at very high pressure after ignition, therefore not as big of valve or port is needed due to the high velocity of the hot gas. the intake can be larger to assist in better flow and velocity. there is a balance between the intake and exhaust that is about 80% of exhaust flow to intake flow. this is measured on what is called a flow bench, this draws air thru the desired port at a standard barametic pressure in inches of mercury and the measurement is given as "CFM" much air has moved thru the port in a give time. the higher the number the more air.

this is the general basic for n/a engines, when any type of forced induction is applied things get much different.

[lapuwali]

If you look at typical cam timings, the exhaust valve usually opens well before the piston has even reached BDC on the power stroke. By the time the piston has reached about 80% of the way down the stroke, no more energy is going to be transferred to the crank by the still-expanding mixture, mostly due to geometric reasons (the big end of the rod is now moving as much sideways as down), and for simple thermodynamic reasons. But, as stated, the pressure in the cylinder is still quite high, so opening the exhaust valve early helps to evacuate the cylinder that much faster.

With perfect timing (which, with any fixed valve timing, will only happen over a narrow rev range), you'll actually have a slight vacuum in the cylinder by the time the intake valve opens while the piston is still rising. The inertia of the exhaust gases will draw the pressure down below atmospheric, and will help to draw the incoming charge in through the intake valve. The additional vacuum created by the piston falling will help, as well, though the intake vacuum is still far smaller than the starting pressure of the exhaust gasses just before the exhaust valve opens.

Since the "force" pushing the intake mixture into the cylinder are so much less than the forces expelling the exhaust gasses, and since you have a limited amount of space to use for valves, you bias the valve sizes towards the intake side.

Again, when the timing works perfectly, the inertia of the incoming charge will allow the cylinder to be pressurised slightly ABOVE atmospheric pressure, and you can shut the intake valve at just the right time, before the charge reverses direction and tries to flow back out the intake valve. This is why you can get such big power gains with just the right cam, but only over a narrow powerband. At the very extreme ends of this were engines like the Honda RC115 4-stroke 50cc GP bike engine of 1965, which made as much as 17hp (340hp/liter, or more than a modern F1 engine), but had a powerband so narrow it required a 17-speed gearbox. Riders talked of needing to make 9 gearchanges in a simple chicane.

[Brett W]

There is no vaccum in the cylinder. Nothing sucks air into the cylinder. There is a low pressure area created by piston movement. By forcing the air out of the cylinder on the exhaust stroke there is a low pressure area left when the piston moves back to BDC. This low pressure area is much lower than atmospheric pressure. When the intake valve opens pressure in the intake tract will force the mixture into the cylinder. Ram tuning occurs at certian rpms when the wave created by the opening an closing of the intake valve bounces back and forces more mixture into the cylinder. This only happens at certian rpms. Look at the old Chrysler manifolds from back in the 60s.

The NASCAR boys have done quite a bit of testing to deterimine valve sizes and such. They have been increasing the intake size and skrinking the exhaust until they can run the smallest exhaust valves possible.

Look at a diesel engine. They are perhaps the most efficent engines right now. The lack of a throttle plate of any kind makes for very low pumping losses. The high compression ratio helps also.

[messix]

i guess that vacuum assist power brakes don't exist? and the vacuum gage used to tune carbs? the low pressure created by the engine on the intake manifold is referred to as vacuum. [not in the absolute term]

[r_towle]

So, taking this knowledge, how can we apply it to analyse the current stock cam profile and how is effects the djet FI system..

Mainly, in a higher HP motor, there is more lift and more duration, and overlap.

I know its all in the combo....

But the question here is what is the true restriction on the cam in a stock FI system, and why.

Without a dyno in my garage, I would like to understand the settings of the FI cam and how they can be changed, and what will happen once these changes are made.

I understand that the MPS is looking at that vaccuum/pressure inside the plenum to determine how much fuel is needed,

With a different sized valve setup, and a different cam profile, it seems that you could maintain a proper Air Fuel ration while increasing the airflow through the system..

The trick is to keep the MPS happy and within range.

Lets say that the MPS is designed to read only a certain set of readings, anything outside that pre-determined set of numbers will make the car run lean or rich.

So if we increase head flow, the car will run lean,,,outside of range of the FI system...

But, if we increase airflow, and increase overlap, we might be able to maintain the same pressure inside the plenum...now the issue would be that the cylinders need more fuel....this could be done with larger injectors and increased fuel rail pressure.

Does this make sense...

I am trying to look at the cam profile, or a change to the cam profile...

Possibly a better flowing head, and or larger valves...

What are the effects, and where can the djet system be taken to...realistically.

Let me give an example, merceded has a 4.8 liter djet driven motor from the 70's....taking four injectors, one could in theory build a 2.4 liter and run it with the proper air fuel mixture.

The readings would be outside of the range of the MPS.
now, can we change the cam/valve size/head flow characteristics to bring the manifold pressure back into range, or is the pressure going to increase or decrease with more air (larger pistons or stroke...)

Rich

[lapuwali]

The cam in the stock engine was chosen more for emissions reasons than power reasons, so it's unnaturally restricted.

There are two problems with using D-Jet with a wild cam. One is that wild cams set up substantial intake pulses in the intake tract, which cause the manifold pressure to fluctuate wildly at small throttle settings. You'd have a very lumpy idle and very poor drivability at or near cruise. You can fix this to some extent by using a mechanical damper (some people use a fuel filter, others use a small restrictor in the vacuum line), at the cost of response. The other problem is the MPS isn't actually measuring airflow. It's estimating airflow based on manifold pressure, air temp, and engine speed.

Any engine (or air pump) does not pump with 100% efficiency. When the intake valves close, the charge pressure will be some percentage of one atmosphere (the volumetric efficiency, or VE). The actual amount will vary based on how far the throttle is open, the air pressure outside, the engine speed, the cam timing, the valve sizes, and the efficiency of the ports. The VE curve will be pretty much fixed for a given engine, so if you know some of the outside parameters and the VE curve, you can estimate airflow, and thus how much fuel you'll need.

Putting in a wilder cam, or cleaning up the ports, or installing bigger valves, or making the engine bigger, will change the VE curve. So, while you still know manifold pressure and engine speed, the VE curve in the D-Jet ECU is no longer correct, so the right amount of fuel isn't delivered. If you can change the VE curve programmed into the ECU, you can adjust this for the new engine.

Note that L-Jet doesn't have this problem, because it measures airflow directly. Thus, it's possible to install a hotter cam in an L-Jet engine and it will work. There's still a potential limit here, as the intake pulsations can confuse the L-Jet airflow meter, so very wild cams can't be used with L-Jet. You can install L-Jet on a bigger engine and it should work just fine. The limit here is the size and airflow capacity of the airflow meter itself.

[Sammy]

Brett nailed it, there really isn't such thing as vacuum. Same goes for centrifugal force.
They are terms that generically decribe a condition in layman's terms, but try and use them in front of a physics professor and get ready for a lecture. Ask me how I know that (IMG:http://www.914world.com/bbs2/html/emoticons/chair.gif)

Neither of those are actually physical properties.

[lapuwali]

IMHO, Brett didn't "nail" anything. Both of you are just being pedantic.

Yes, airflow will always move from a higher pressure to a lower pressure. Reducing the pressure in the cylinder below the pressure of the air in the atmosphere will induce flow from the outside, through the intake tract, into the cylinder.

In common use, "vacuum" is simply "pressure lower than atmospheric". The fact that pressure pushes rather than vacuum sucking is simply idle semantics, and makes zero difference to the actual operation of the engine. You may as well go around constantly correcting everyone who misuses the terms "torque" or "force" or "work".

Describing things in "layman's terms" is USEFUL, and was exactly what was asked for by the original poster. Indeed, if you can't sufficiently explain something in non-technical terms, either you don't really understand it, or you're an inarticulate Neaderthal (hey, would you like to beat me up on the them "Neaderthal" now?).

Neither one of you have actually added to the conversation. Brett's statement: "The NASCAR boys have done quite a bit of testing to deterimine valve sizes and such. They have been increasing the intake size and skrinking the exhaust until they can run the smallest exhaust valves possible." My response would be: fine, WHY? Things are the way they are because the NASCAR boys say so? What's the theory behind this?

[lapuwali]

Easy. Explain the technical terms first in layman's terms. This is the essence of teaching. Build on existing knowlege to add new knowlege. One also hardly needs to rely on exact terminology to understand something. Simply knowing terms is a fairly useless thing. Knowing the concepts those terms represent is the useful bit.

[r_towle]

QUOTE (James Thomlinson @ Aug 16 2005, 07:32 PM)

Disregarding the automotive discussion for a moment, I think what you state in your second to last paragraph is incorrect and contradicted by what you said in your last paragraph. In science and engineering if you want to provide someone with a true understanding, you will have to "get technical". For example, how would you explain electricity and magnetism or light in non-technical terms?

electricity is like the waves of an ocean.
or the current flowing down a stream
The width of the stream is the voltage, the amount of water.
the speed of the water, the force, is the amperage.

Magnetism is the same thing as love or lust or the need to be close to someone.

Light is the opposite of dark.
Dark is black, light is white.

[rhodyguy]

no electricity, eye can see no light, my big toe(flesh) is like a magnet to the foot of the bed(wood) . (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) i try to limit my criticism to their, there, they're, where, wear, we're.

humorless electus (k)

[r_towle]

Back to our topic.

I believe that djet could be fooled.

If the VE is hard coded, its saying that given air temp, air flow and throttle position, the system will pulse the injectors for a certain amount of time.

Break that down.
Air temp can be fooled with electronics.
Throttle position can be fooled.
The one that I am having a hard time with is how to fool the MPS.
there must be a way.

If you bump up the fuel pressure, and put in larger injectors, you could add more fuel for a given airflow..

Here is my driving question...I am not looking for a wild cam or huge valves or anything ridiculous.

I am thinking 2.2 liters, a bit more of a cam that will keep it drivable (this is where I need help)
And still retain the original one second start of the djet that I love.

I know that 2056 has been done just be adjusting the MPS....
Now, go to 2.2.
Change the electrical output from the MPS versus how it is reading its input, normalize it to its original output.

Port the heads, slightly larger valves possible (this would depend on the cam etc...)

Just look for 150 hp from the original djet,,,,I am sure that Jake could make a 2.2 liter get close to 200 hp, that is not the idea.

Djet took a certain motor and would run it contricted for both economy and long engine life...

I am trying to figure out how to do the same thing at the 2.2-2.4 liter range...run it conservatively..

A 2.2 or 2.4 liter making 150 hp could be a nice long life motor with a nice torque curve and good around the town fun.

I love the single turn to get djet going.
I have plenty of spare djet parts.

I also have two Ljets, and going to your last comment, I know I can do this with Ljet,,,,just not sure I want to go that way yet....I might...but I have not real world experience with ljet, what I have read it can be done, has been done, and is still being done...

Rich

[rhodyguy]

such A thing as vaccum? i beg to differ. my dad was so frugal you had to force a cent out of his pocket. he was very centafrugal and he existed. is that what you mean?

a poor child (k)

[lapuwali]

You can fool the MPS. Indeed, you can replace the MPS with an entirely modern solid-state unit that will outlast the tub. However, to do all of this, you require enough electronics that you're 80% of the way to an full fuel ECU. If you used (nearly) any aftermarket ECU, you can replace the stock ECU, the MPS, and use all the rest just as it sits. You'll have a fully programmable fuel curve that you can tweak to suit any Type IV you can dream up. It will also have the easy starting of the D-Jet system.