P Mack

iTrader: (6)

Also maybe this ve definition stuff should be in a new thread.

'JustDreamin'

Funny, but the original poster didn't ask anything about volumetric efficiency.

He was asking about fuel economy, which, unless we're talking Nextel Cup fuel economy, is usually achieved at part throttle, which absolutely kills volumetric efficiency. So it really doesn't matter how we define VE.

Given the arguments over VE, I think it would be good to have a thread that explains VE properly. But that should be another thread.


I would certainly be interested in this thread actually addressing fuel economy (like the poster asked) and what conditions are favorable for high fuel economy. Ideal lean mix, timing, pre-ignition, detonation, high EGR rates, water injection, etc. What makes for the best fuel economy (besides less vehicle weight / aerodynamic drag / rolling drag / engine displacement)?

'JustDreamin'

technical

When I tune my car for cruise I tend to pump the timing up pretty far. I logged timing recently around 36-38 with IAT's ~50* no KR. This gives the car a punchy feel in higher gears (4th & 5th) which allows you to cruise without bogging. I don't have EGR since my car came with a LS6 intake, but EGR does help fuel economy by cooling down the cumbustion chamber and keeping pre-ignition at bay. I don't run lean cruise.

The original question of operating most efficiently at the verge of pre-ignition is easily answered by understanding that the point of optimal efficiency (where the temperature and timing creates the most complete burn i.e. power) is a degree or two before pre-ignition. If the timing is too far retarded the mixture won't burn as complete resulting in less power and higher combustion temperatures and possibly more pre-ignition. It's a balance between temperature (resulting from compression ratio), timing, fuel mixture, etc. You want the most heat from the power stroke but the least heat during the other three.

ringram

You want max pressure at 14 ATDC as per J-Rods thread (not heat), the thread which I think is a good start for some of us.

On the economy side, guys in Oz where the holdens run lean cruise say the stock AFR of 16+ isnt good as you need more throttle to maintain speed. An AFR or around 15.4:1 at part throttle, low load gives best economy. I agree timing will be in the early to mid 30's at this point based on most of the maps I have seen for lean cruise. I guess you could lean up the idle and drop the rpm's as well. Leaner is ok for economy and power, but bad for the engine due to heat and extra risk of ignition issues.

technical

Since pressure is directly proportionate to heat, there is a correlation. Static compression ratios do not change within the engine, but temperature does. Since temperature is a variable, it affects the combustion pressure. That's why your car might not ping at 50*F but does at 90*.

jimmyblue

iTrader: (11)

The earlier you start the burn, the more time it will be
contributing energy to the crank. The delivered work
is the integral of pressure(t)*piston_area*
sin(rod:crank_angle(t)) or something like that,
which is a messy thing, but anyway earlier is better,
to a point. So early that you fight the end of the
compression stroke substantially, costs you (and is
the best way to get detonation, spark against a
rising instead of falling cylinder pressure). You don't
want a bunch of extra cylinder pressure left at BDC,
you want to have taken all the heat out as expansion
which means you want it to have started off as
compressed as practical.

Lean for economy and rich for power both are playing
off efficiency against shortage. For some distance from
stoich, you can improve power by enriching because
you gain more from burning more fuel inefficiently, than
from burning airflow-limited fuel more efficiently. Airflow
I think is the deal. Similarly on the economy side you
can use less fuel, burning fuel-starved with some lower
efficiency than burning the "correct" amount. Eventually
on either side efficiency starts to go down faster than
the benefit of more/less goes up. Competing mechanisms
produce a rich and a lean optimum (with some fairly flat
land about it; pic).

"Load" is correlated to the cylinder charge which means
the compressed cylinder pressure which drives the burn
speed of the charge. Low load, low pressure, slow burn
allows / wants more "lead timing" to get the pressure
peak after TDC. Same mixture, higher pressure, same
timing, detonation (burn before TDC).

treyZ28

I've got to play around some 135VE NA engines. mmmm sexxy. Isn't F1 creeping on 200?







Anyway this thread is a clusterfuck of erronious info (not in regard to dano).
Best (lowest) BSFC in a 2.0 liter 4 cylinder will come at about 1/2 throttle and 2700rpm or so. crazy, huh.


http://me.queensu.ca/courses/MECH435...erformance.ppt

pg 26 or somewhere around there.

EdmontonSS

The "VE" definition debate has been added to here:
https://ls1tech.com/forums/advanced-engineering-tech/419053-official-volumetric-efficiency.html
I'm not saying the VE calculated relative to atmosphere is "wrong" per-say, just that it's only one definition of VE from thousands, and probably the least useful for the purposes of most technical discussions.

'JustDreamin'

I'm trying to decode what you've written above....

BSFC (Brake Specific Fuel Consumption) is a measure of the fuel required per horsepower. Here a lower number is better. A lower number indicated that for a given amount of fuel you get more shaft horsepower out.

I saw 2 charts in that link.

1st chart said that BSFC goes down as engine displacement goes up, which makes sense. As displacement goes up, there is less surface area relative to the volume enclosed, which means there is less heat (in the burning air fuel charge) lost to the cylinder walls and is therefore available to produce useful work. So, bigger engines (assuming that you're not getting bigger strictly by adding cylinders) have the advantage.

2nd chart said that BSFC is lowest in about the middle of the rpm range and goes up as you get towards idle & redline. This makes sense for most production engines, which have camshafts that are probably the closest to ideal in the 2500 to 3000 rpm range. Above and below that rpm range, the camshaft is less than ideal, and the cost is increased BSFC (more fuel used per delivered hp).

'JustDreamin'

treyZ28

sorry, "Max" I meant "best." that is a bit misleading now that I look at it.

Lower is usually due to increased time for heat in the cylinder to be absorbed into the cylinder walls. Higher is due to friction.

Typically, unless its a radical cam- the cam plays little role as long as it is within an operating range. Its hp vs fuel usage. So it doesn't matter if you make less power, it just matters how efficient the engine is at that rpm.

Bore, stroke, compression, spark timing and rpm are much bigger factors.

treyZ28

No, there is ONE definition of VE. It is not negotiable.

BigBronco

iTrader: (2)

jesus...


by the way, there is no such thing as detonation in a motor only "Knock" Detonation is something that is completely impossible to happen in an internal combustion motor. They also discussed and confirmed this at the SAE convention prior to the PRI show...

Adrenaline_Z

I think Jesus might want to read this:

http://www.streetrodstuff.com/Articl...tion/index.php

Detonation, or knock, or whatever you want to call it...is the name given
to uncontrolled combustion which causes sudden spikes in pressure within
the chamber.

The 'knock' is the pinging that is picked up by a frequency sensitive sensor
which reports back to the computer.

technical

What we all hear when our engines ping or knock is combustion taking place too soon but after the spark plug has fired. Not necessarily a second burn of end gases. If the mixture is fired off too soon and propagates before the piston reaches TDC it pings. That's why the PCM retards the timing. According to that article, we would call that detonation since the plug has fired but it isn't a second combustion of end gases so it doesn't quite fit their definition. I was taught that scenario fell under pre-ignition since combustion occurred too early even though the spark plug fired. A hot spot caused by a formation of carbon can do the same thing. Either way it's that scenarion that causes all the damage.

Adrenaline_Z

OK.

Are we reading the same article? What page did you find otherwise?


I have a few books and charts that show detonation spikes which occur
after the plug fires and after TDC. I don't recall that link being any different.
The author specifically says deontation occurs after the spark and while the
piston is going downward.

Adrenaline_Z

Here are three other sources to confirm detonation occuring after TDC:

technical

I was referring to this section...

Those statements do not include TDC as a qualifier. I agree with those statements and yours that detonation occurs after TDC, but the scenario that I describes happens after the plug fires but before TDC. What I am on the fence about is the terminology.

The article describes pre-ignition...

...which would be true for the scenario we most commonly deal with (the one I described). The same reason a diesel pings.

APeteSS

It's mass, volume is limited to the displacement of the cylinder. Mass changes with density.

From what I've learned (some from EFI 101 basic & advanced, other from experience) there is a "sweet spot" where maximum controlled cylinder pressure is desired. This "sweet spot" is around 15 degrees past TDC which allows for the energy released in the power stroke to do the most work (think leverage). Much like pedaling a bicycle, less energy is wasted in trying to expand a volume that isn't ready to expand yet (like when firing at 12 and 6), but well-timed maximum cylinder pressure just after TDC is optimum. The "sweet spot" is the point where maximum (controllable, I'll address this later) cylinder pressure is desired in the cylinder, assuming that it does take time for the pressure to build (known average is about 100 ft/sec), attaining this means finding the correct moment to fire the spark (usually before TDC, and even sooner as RPM increases). This is why more efficient heads and engines generally require less timing than their less-efficient counterparts, a CC that allows a fast flame front doesn't need to begin that process so early in order to arrive get max pressure at the same point after TDC. Larger bores, lower compression, higher octane, etc. also generally require more timing.

As described earlier, detonation before the desired point in the crank revolution, whether caused by deposits, IAT, fuel, too much timing, etc. is undesirable because of the intense pressure spike created by the detonation itself, along with the fact that the mixture is trying to force a power stroke while still the engine is still on an intake stroke-- too much of this will take it's toll on everything that serves to seal a cylinder and soon enough, you'll find the weakest point. EFI 101 has some great intra-cylinder photography showing flame fronts at different crank angles and head designs, great to help conceptualize the mixture burning process.

All of this my take on the current topic-- thats it, feel free to comment.

Adrenaline_Z

Technical,

Pre-ignition occurs before the spark plug fires. The mixture ignites from hot
spots in the cylinder. The hot spots could be a glowing sliver of metal on the
edge of the cylinder head, or a very hot spark plug (wrong heat range), among others.

The mixture begins to burn/combust while the piston is moving upward (much
before the spark plug was to fire). The mixture expands in the volume created
by the piston rising toward TDC and the cylinder head chamber.

If the rate of expansion is faster than the piston is able to reach , or pass TDC,
the pressure created by the mixture will try to press the piston back down
(as the piston tries to move up).

This of course creates a huge loss of power due to counter active forces, and
it will present major damage to the mechanicals.

Detonation on the other hand may/could (?) occur before TDC, but since it happens
after the plug fires, the piston is able to get around TDC and begin moving down before
the mixture presents damaging opposing pressure on the piston crown.

Just think that the mixture takes time to react and begin to combust; also
keep in mind that the piston is moving at great speeds. The time intervals are
measured in milliseconds.

So by definition, what YOU are talking about cannot be pre-ignition because
it is happening after the spark plug fires.

I've studied diesel technology in short bursts. I can tell you that diesel ignition
has nothing to do with ping. It uses extremely high compression and high
pressure injection systems to present an air/fuel mixture into the chamber at the point when compression can cause ignition through heat.

mdrew

Do you guys always argue like this? You’re worse than my project engineers…..

You are all saying basically the same thing, but including variables that apply to both sides of this argument. It would appear to an outsider that you are simply thinking too hard. In other words, you’re acting like engineers.

I’ve always understood it like this, in very simple terminology.

Detonation: An Erratic form of auto-ignition caused by excessive heat and cylinder pressure. Instead of a single point of ignition (plug), there are several ignitions sources created multiple flame fronts typical to spontaneous combustion. These multiple flame fronts collide creating an explosive peak in cylinder pressure (shock waves). These peaks are what you hear as a metallic pinging. Detonation can occur prior to, or after the plug has fired depending on cylinder pressure and heat. Diesel engines use detonation as a means of continued cylinder firing after the initial firing via glow plugs, which is then controlled by injector timing relative to piston position. Retarding the cam and using higher octain fuel have always eliminated this for me. Or, more overlap.

Pre-ignition: Another form of auto-ignition, but occurs when cylinder pressure and heat create a ‘hot spot’ that acts like an ignition source and ignites the air/fuel mixture prior to the ignition spark. – Hence the “pre” part of pre-ignition. Pre-ignition generally occurs prior to the piston reaching TDC which is why it is much more damaging than detonation. Causes of pre-ignition can include; A bur on a valve or valve relief, too hot of a plug, carbon deposits. I was taught to reduce the chances of pre-ignition during assembly by polishing everything in the combustion chamber and rounding off all sharp edges that could become sources of ignition.

Maybe I’ve been wrong all these years??? Damn, hope not…..