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volumetric efficiency

3076 Views 4 Replies 2 Participants Last post by  reefedjib
Now that my incursion into the realm of the short block has reached a natural stopping point, I have become aware of the complexity of the flow of air/fuel through the cylinders, related to carbs, manifold, head, cam params (lift, lash, open/close angles - i.e duration), valve size, port size, port shape, quenching, and exhaust. I don't think it affects the compression ratio, but higher cylinder pressures, early or late, depending, would compress to a higher compressed pressure. Dunno how this affects VE - does air/fuel burn better at higher pressure? I believe this is the volumetric efficiency. What is a good cylinder pressure target for N/A L28 engines? Heck, what are the units to describe cylinder pressure!?

Could anyone talk a bit about this and help educate me? Many thanks.
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Suppose you have a cylinder with 100cc displacement. As the piston moves, it draws air in through the intake valve. As it gets going faster, you can measure a partial vacuum in the intake manifold, since the air flow will be restricted by the air cleaner, manifold, runners, etc etc. Because the airflow is slightly restricted, and because the intake valve itself constricts the flow, at high rpm's the cylinder is not able to completely fill before the intake valve closes again. The higher the rpm's, the harder it is to fill the cylinder before the valve closes again. So if, for example, the cylinder only gets 70cc of air at 5000 rpm's, the VE is 70%.
Of course, valve timing and overlap, air cleaner design, manifold dimensions, valve size and lift, and about a million other details all effect VE. But VE is itself a very simple concept.... VE= (actual air taken in / actual displacemen)x 100
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Thanks ghosttanker. If the VE drops as rpms increase, is this what causes the torque curve to peak at a particular rpm and then begin to fall off?

I have heard of badly tuned or mismatched intake components completely losing power at say 5000 rpms, is this what is causing that, as a more drastic form of the torque curve falling off?

What is the relationship between VE, cylinder pressure, rpm and torque?

I can kinda see how the air cleaner, carb venturis, manifold and runners will all work to produce partial vacuum and a restricted airflow. I am curious about the impact the cam has on this, particularly on the intake side. Valve size and lift relate to flow restriction. But what about open angle and duration? I heard something along the lines that an advanced open allows for higher VE, but that the cylinder pressure increases later, which is better. A later open creates an earlier cylinder pressure?? (this makes no sense to me so I am just jabbering it out here!)

many thanks.
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Like I said, cam duration and overlap affects VE. Overlap is the period of time where the intake and exhaust valves are both open. During overlap, the momentum of the exiting exhaust gas helps pull fresh air/fuel from the intake side. This is known as "scavenging". As rpm's rise, more overlap helps performance, but at low rpm's, overlap causes intermingling and rough idle...the traditional"lumpy cam" idle. The diameter of intake and exhaust runners affects gas velocity, which also affects scavenging. A long, narrow runner has better scavenging at low rpm's but at high rpm's becomes too restrictive to flow. In addition, a sonic phenomena occurs inside runners, which causes standing pressure waves to form at various rpm's. Runner lengths can be "tuned" to enhance this effect, and the result can sometimes be so effective that, in a certain narrow rpm range,the harmonics of the intake and exhaust provide such effective scavenging that VE can exceed 100%. All these factors (cam overlap, runner length, diameter)and others, combine into the black art of engine tuning. The unfortunate reality is that, on a relatively primitive motor like the l-28, highest VE's at high rpm's occur in a relatively narrow rpm band, and you have to build the motor to match what range you want to operate it in the most. You can build a torque monster for low rpm grunt....long runners, less overlap, smooth idle---or you can build a screamer high rpm motor....short, large diameter runners, lots of overlap, and a high rpm sweet spot, that when you hit it, sounds like the joyous blowing of gabriel's trumpet invoking the second coming of the high gods of acceleration. If you want a motor that does both, you need a modern Honda or BMW thing with variable valve timing, dual-plane intake manifolds, and complex engine management systems that provide direct injection and all sorts of other gee-whiz complications.
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cylinder pressure and compression ratio related to torque

theghosttanker said:
and a high rpm sweet spot, that when you hit it, sounds like the joyous blowing of gabriel's trumpet invoking the second coming of the high gods of acceleration.
LOL! Awesome! I am totally up for the trumpets and not so much the torque grunts. Sonic harmonics must sound symphonic - that would be nice to have.

If you want a motor that does both, you need a modern Honda or BMW thing with variable valve timing, dual-plane intake manifolds, and complex engine management systems that provide direct injection and all sorts of other gee-whiz complications.
I am very, very pleased with the prospect that I will have no computers on my Z. I don't have a cell phone, either. Sure there is big benefit, especially with the things you mention, but I want to go old school since the way I earn my living and the way I spend much of my free time involves computers. Oh all ye Luddites! I need a primitive escape.

EDIT - I spoke with a guy at work, who drives a 2008/9 BMW 535i. He says its an inline 6 turbo with 300 hp. I asked about variable valve timing and he said he didn't think it had it but it used to. He said his year they switched to direct injection. I just checked the beemer site and the current year 535i has both direct injection and variable timing (and twin turbo). Those are definitely complications, but I suppose they are useful. The computerized control of the Otto cycle.

1) What is quenching? Found it:
The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power.
EDIT - Ha! I found oodles of good info here: http://www.carcraft.com/techarticles/horsepower_vs_torque/index.html. You gotta love the internet!

regarding my second question:

2) Will the same displacement engine produce the same torque at the same rpm, if the CR is different? Or does this higher compression result in a higher torque at the same rpm? (bigger force, same leverage crank distance, same rotation - so it can drive a bigger dyno load). I just have this gut instinct that higher CR burns more...kinda related to the next question on cylinder pressure.

Compression Ratio

Much like increased engine displacement, higher compression ratios are a sure path to increased torque. The overriding factor is, of course, fuel quality and detonation. There are numerous factors to consider here. Finer atomization of the fuel and more precise control of air/fuel ratios via electronic fuel injection has allowed O.E.M. manufacturers to increase compression ratios above 10:1 in some late-model, high-performance cars. The very latest LT4 Corvette engines are actually sneaking up on 11:1 compression ratios again because of the inherent efficiency of electronic controls and the combustion-efficiency gains made in the cylinder heads and induction system. Carburetors are less precise, but there are other ways to increase torque with higher compression in carbureted engines running 92-octane gasoline. Many street engine combinations running a big cam for top-end power experience a significant loss of low-end torque. This occurs because the intake valves close much later when the piston is farther up the bore. Thus, the dynamic compression ratio is less than the theoretical compression ratio that assumes full-stroke piston travel. If you are going to run a big cam, one of the bonuses is that you can increase the compression ratio slightly without incurring a detonation penalty. The increased compression will boost the low-end torque and extend the top-end power range. Experienced engine builders have found that 9:1 compression engines require at least a 270-degree (advertised duration) cam. On the other hand, 10:1 engines are happy with a 280-degree cam, and a 290-degree cam will allow you to run nearly 11:1 compression. Depending on other engine variables, such as combustion-chamber shape, bore diameter and ignition timing, some engines will detonate under these conditions. In these cases you need to go to a smaller cam or run slightly less total timing. In any event, the idea is to use as much compression as possible relative to the cam profile in order to gain low-end torque without detonating.
and regarding my third question:

3) what is this deal with cylinder pressure? Whatever the CR, say 10:1, it sounds like a different pre-compressed pressure (which would be a different 10X compressed pressure), will run differently. Ok, more air/fuel mix to ignite, so more energy/force released, at least this is how I think of it. I would expect a Turbo to raise this pressure, which may be why a Turbo engine has a lower CR. The guy telling me this observed that different valve timing causes different cylinder pressure at different points in the stroke cycle and that this affects performance. I did not have the opportunity to explore this further with him. Could you explain the deal with cylinder pressure and how it may relate to VE?

Camshaft Timing

When you consider valve-timing events, you also have to consider all the other elements acting on the fuel charge and combustion gases in the cylinders. An earlier-closing intake valve starts building cylinder pressure sooner. This increases low-speed torque due to greater cylinder pressures, but it means that the engine is having to work harder to compress the charge. As previously explained, a later-closing intake can enhance top-end torque at the expense of low-end torque, but you can get most of the torque back on the low end with an increased compression ratio. What you look for is a cam profile that promotes increased cylinder filling with earlier intake opening so that the valve is farther off the seat during the early portion of the intake stroke. Then you want to delay the exhaust-valve opening as much as possible to take advantage of all the energy you can from the combustion process before you blow down the cylinder. A quick-opening exhaust valve is helpful here, but, again, there are trade-offs.

This combination builds good torque but tends to increase valve overlap at TDC. This is where the cam lobe separation angle takes control. The lobe separation angle is the angle between the peak of the intake lobe and the peak of the exhaust lobe expressed in cam degrees. Tighter lobe separation angles (less than 110 degrees) make more torque and horsepower, but, with more overlap, the engine experiences poor idle quality and high fuel consumption. Opening up the lobe separation angles (more than 110 degrees) broadens the power band while improving idle and part throttle characteristics. With these wider lobe separation angles, peak torque and power are generally reduced, but the engine becomes very smooth and drivable.

Most street and high-performance engines will perform best when overlap is between 35 and 70 degrees (measured from intake-valve opening to exhaust-valve closing) with the duration as short as possible within the overlap guidelines. If you choose 50 degrees as a middle-of-the-road overlap figure for a pretty hot street machine, the shortest possible duration with this overlap will produce the most torque. You could make more torque with a bigger cam--but only at the expense of driveability and economy.
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