combustion

Must be running some low HP engines. Mine is 250 PSI pre-ignition. You have to be careful about pinpointing an exact ignition point. A Diesel uses heat of compression to light off the fuel, depending on the quality of the fuel being used, there is a range that the event does its thing. Fuel quality, air density, moisture in the air, ambient temp, on and on and on. The limiting factor is the flash point of the fuel. Compression calcs will approximate piston position at point of ignition.

123, thanks for the info. I should have seid psi afte ignition. All I know is ,a normal engine has a 9:1 ,10:1 or 11:1 compression that is equal to 129 ,143 and 157 psi. Your 250 psi engine must be a racing or a low compression diesel engine because you have a 17.5:1 compression. I just want an average number of psi when combustion takes place.

Your figure is about right for a cool spark ignition engine. At the instant of ignition, the pressure is no higher. Then it begins to rise as the heat of combustion elevates the air temperature in the cylinder. As the piston goes over top dead center, the pressure continues to rise, and one concern for engine design is to apply the rising pressure in the most beneficial part of the stroke (by getting the burn started about 30 degrees before TDC) re the leverages involved. Start too early, and pressures rise too fast, impeding the piston's rise, and favoring detonation; wait too long that the temperature begins to decline too soon from expansion on the down stroke. If you google for BMEP, you'll find lots on brake mean effective pressure, which is the pragmatic way of looking at this: it's the average pressure pushing the piston down.

Instantaneous pressures can be calculated, but doing so is not simple, by any means, because nothing is staying still, and the difficulty is compounded by the interrelatedness of reaction rate and pressure and temperature, all of which refuse to hold still. The only pressure that is easy to come close on is the one you mentioned, at the instant of ignition, which is essentially the same as at the instant before ignition.

If your interest remains after googling there are entire texts devoted to combustion in ICE's. Other google terms: "causes of detonation", BSFC engines, torque BSFC, engine "volumetric efficiency", CFD combustion.

http://www.plcs.net/

Sorry don't know how this got in the wrong thread.

If you got your 140 psi or is it psig at top dead center, that is maximum cylinder pressure, that is not the pressure at the point of ignition which occurs before the piston reaches top dead center. Therefore, if ignition occurs before top dead center, re-computing based on the cylinder volume when the crankshaft is at the point of ignition will get you closer to the number I think you want which won't be too far away from the pressure at TDC. If you're looking for cylinder pressures as a result of combustion, I think there is another post with a link to BMEP which should give you more information.

The question is do you require compression pressure, or firing pressure.

Generally we do not have provision in petrol engines to record firing pressures. By the way the flash point of a petroleum based product has no reflection on engine performance, it is only a measure of the storage risk of the product.

With distillate, in a CIE, it is impotent for combustion engineers to know the Auto Ignition Temperature of the fuel.

Guys, I don't have the facility to do so but if I mount a pressure gage on top of the cylinder heat , turn on the engine it should give me a pressure reading after ignition . Increasing the RPM should also increase the pressure . If I idle my 6 cylinder at 940 RPM then step it up to 4000 RPM how do I figure out what the pressure is after ignition . Please I am not interested in flash points , 30 deg angle before ignition or after. Just a formulae or an average number will do. If nobody knows I'll consider this subject closed.

Thank you for all of your input.

From your post #3 it looks like you're assuming isothermal compression from atmospheric pressure. It isn't, pressure rises more and temp also rises - essential for diesel engine ignition.

Also I believe these pressure indicators screw into the plughole so there's no ignition (on that cylinder) so can't measure pressure post ignition.

Somebody correct me if I'm wrong, but I'd expect the the actual pressure at idle to be much lower as the throttle is nearly closed and the cylinder pressure at start of compression is well below atmospheric. Also I wouldn't expect it to rise much if revs increase to 4000 on no load, because on no load it only takes a very small increase in throttle opening to increase the speed. If above is right, it raises an interesting question - how are these things used? I can't remember last time I did it. I suppose you could disable ignition in one of a no. of ways and crank the engine with throttle wide open. Anybody enlighten me?

FKIA mentioned BMEP (I assume the link explains it) which gives a clue to pressure after ignition. BMEP is the output power divided by the flow throughput (in the right units of course). Flow throughput is engine capacity x rpm/2 if it's a 4-stroke (I'm not sure you divide by 2 for the definition of BMEP, but you do to get the right pressure. Otherwise need to multiply by 2 later). Pressure just after ignition of course is higher, but from BMEP and relative volumes of combustion chamber and cylinder (which you know from compression ratio) it can be estimated.

Codey

Hi Codemaster, re your comment:-

"Also I believe these pressure indicators screw into the plughole so there's no ignition (on that cylinder) so can't measure pressure post ignition"

The compression gauges are held in the spark plug hole by hand in a petrol engine, and the engine is turned over on the starter motor, however do not try this with a diesel engine because your compression ratio can be anywhere from 14:1 up to 25:1. On smaller diesel engines the compression gauge is clamped into the injector hole, in place of the injector. On larger diesel engines a cock is screwed into the cylinder head and a compression/firing pressure gauge is screwed onto the end of the cock allowing either compression pressure or firing pressure to be taken. This is usually carried out 1 cylinder at a time and the engine is acually started.

Compression pressure will not alter very much, as a matter of fact, it will decrease slightly as engine rpm is increased, due to a falling off of volumetric efficiency (Always less than 100%). After all compression pressure is conrolled by the compression ratio. (It will also alter in respect to engine codition as well)

This only applies to naturally aspirated engines.

OK MOBI, thanks. Also thanks to Guest #10. My comment about wide-open throttle of course wouldn't apply to CI engines.

Cheers....Codey

True, the pressure rise is increased by the rise in temperature, True that a spark-iginition engine on light load won't read anywhere near such a high pressure (if you've the gear to measure it), however if its compression-iginition there isn't a throttle and the cylinder always fills with air at whatever the inlet pressure is (near atmospheric if not supercharged).

On the compression tester instrctions it usually reminds you to open the throttle fully when doing the test - well that was when everyone had petrol engines except the hauliers and corporation bus garage.

codemaster ,please read my question correctly.First paragraph,isothermal compression. Atmospherig pressure is about 14.3 psi ,compressing any amount creats heat , so what I dont care. Second paragraph, obviously you can't check combustion pressure with the spark plug removed. I ment drilling -tapping a hole in the cylinder heat and screwing in a pressure gage, then turn on the ignition and read the pressure. Third paragraph, you can't have below 14.3 psi atmospherig pressure that is why a 10:1 compression gives you 143 psi pre-ignition pressure. At a higher RPM more fuel is burnen therefor the pressure at combustion will raise too. Disable ignition and cranking engine with throtle wide open would't give me anything I'm looking for.

Paragraph 4. Not clear to me how to apply information to establish psi pressure after combustion in a piston motor

Johann - your original post asked how you could figure it out, not how you could measure it. That's what I and others were trying to help you do. You only later mentioned drilling and tapping the cylinder head.

But I don't agree with some of your comments. You seem to be under the impression that compression ratio is the ratio of pressure after compression to before. It isn't, it's a misnomer, it's the ratio of volume of (cylinder + combustion chamber)/volume of combustion chamber. I nearly mentioned this in earlier post, but I thought most readers would know. Pressure ratio would be the same if compression were isothermal (Boyle's Law) but it isn't.

Then you say you can't have below 14.3 psi atmospherig pressure. You most certainly can. At idle the cylinders and inlet manifold are at much lower pressure, at a bit of a guess I'd say < 0.1 bara. This vacuum is used to operate brake servos, and fluctuations in pressure are used to adjust ignition timing and other engine management parameters.

Para 4 - if you know the BMEP (as I defined it, may not be "official", as I said) you know average pressure during the power stroke. You know the expansion volume ratio (see above). If you make some assumption about expansion equation (I'd start with polytropic, exponent 0.23) you have 2 sets of conditions between P₁ and P₂ enabling estimate of both. Might not be vey accurate but you seemed to be stuck and it would give some idea. But if you're going to measure it anyway, why bother?

Cheers...Codey

You may be looking for a simple answer to a complex situation. There is no single pressure that occurs after ignition in a spark ignition engine. When ignition occurs, the piston is still on its way up, so pressure continues to increase by virtue of simple compression. In addition, the heat of combustion causes the pressure to increase dramatically. So at full throttle and heavy load, the peak pressure goes to about 1500 psi or more for a gasoline fueled engine, (and much higher for something running on nitro [a graph for which is here]). On the first graph, the dark pink curve is the pressure that would have occurred if the spark had not fired. The blue line is the pressure that occurs with normal nitro combustion, under full load.

But change the load, and all this changes. Peak combustion pressure changes with throttle setting, fuel mixture, breathing effectiveness, scavenging efficiency, etc, etc, etc. So one engine may have a peak of 1800 at a particular load and speed and another will have a peak of 1200, at a different load and speed. At a 940 rpm idle for an average 3.0 liter engine, peak pressure might be not much more that 250 - 300 psi. If you simply increased the throttle to bring the engine to 4000, and did not load the engine, the peak might be 350 psi (maybe even less). Under full load (and therefore wide open throttle) at 3000 (or wherever your engine reaches its peak torque) peak pressure will be in the 1500 - 1800 range.

Both peak and mean cylinder pressures are highest at the engine's torque peak, and fall off at higher speed because the cylinder is not being fully charged. As you can see from the linked graph, the pressure peak is fairly narrow (although a gas engine would be much broader but lower), so the average pressure that sends your car down the road is much lower. BMEP is not a real measured value: it's the pressure that, if applied from the top to the bottom of the each power stroke would produce the brake hp of the engine. BMEP is typically 150 psi for an average modern engine.

Ken you are a real jewel. It gave me the answer I was looking for and much more but also some doubts if my rotary engine design could work. If you are interested we could discuss the basic principle of it.

Thank you.

Direct injection fuel injection engines use 1750 psi pumps, if that's of any help.

Yes, but that's not just to get the fuel into the chamber, it's to get it 'atomised' and distributed into the air charge so is much greater.

Hang on Guest and Wrenched!

Have I missed something?

#1. You cannot specify a fuel injection pressure for a direct injection CIE per se.

Injection pressures could be 3 000psi in a high speed high compression (24:1) or it could be 2 000psi, or 1 750psi.

#2. This is the injection pressure and it is controlled by the injector spring, which is adjustable. This is why injector opening pressures are always checked when carrying out injector maintenance, among other tests.

The diameter of the holes in the nozzle body, the number of holes etc., plus the pressure ensures good atomization and penetration of the atomized fuel into the very dense, hot air during the compression stroke.

Not missed anything so far as I'm aware? I just meant to say that the injection pressure and the compression pressure are not particularly related, except that the first has to be greater than the second.

MOBI, maybe he's talking about the direct injection 2-stroke cycle SIE's that have been going up against the 4-stroke cycle SIE's in the marine outboard motor market these days? I don't know what injection pressures these 2-strokes use. You're absolutely right about the pressures for the CIE's. Cheers...............

Please! Please! Do not talk about outboard engines, I will be unable to sleep tonight.

They have one good use ..............anchors or snapper drops.

I apologise, I was talking about real engines!!!