Desert Dog

Since this is an advanced performance discussion, I would like to start a thread on the search for fuel economy while maintaining performance.

I would like to understand better why an internal combustion engine operates most efficiently at the verge of detonation.

Big_Brother

Not sure I'd agree that an engine is most efficient on the verge of detonation, I'd think it's most efficient at highway speed turning around 2000 rpm (V8)...very little hp being used to sustain a steady speed. Maybe more power output when close to detonation?

In any case the contributing factors that lead to detonation are high combustion chamber temps, low octane fuel, overadvanced timing, carbon deposits/build up, high compression and a lean mixture.

What actually causes the detonation is too high of a pressure in the chamber; in a normal ignition event the flame front travels from the plug evenly across the piston at about 5000 ft per second, when detonation occurs the rate of burn climbs to around 20000 ft per second...way too fast for the piston to get out of the way...the resulting pinging is the pressure waves bouncing off of the piston, cylinder and valves....not good...kinda like lighting off a stick of dynomite in there; the engine wasn't designed to withstand those types of pressures for very long.

So as you can likely surmise; higher pressure = faster piston movement, but get it too high and lose power through detonation.

I'm sure I did not include all causes and scenarios, this is just a brief explanation.

JamesO WS6

Well Big_Brother, Desert Dog is acutally correct. Engines are most efficient right before they detonate. And the reason for that in short is b.c. they are so lean. And we all know that leaning out a car nets you performance gains. But there is a fine line between performance gains and big problems. If anyone here has blown and engine they will tell you that it was running at it's best RIGHT before it blew.

N/A engines typically have 80-85% Volumetric Efficiency. With a leaned out engine the percentage will rise causing great performance gains but severe damage to the enigine. There are however other ways to increase an engines VE while retaining it's drivibility and reliability while still getting the performance. Forced Induction! Supercharged engines usually have about a 100% VE, where a Turbo can have 110%+.

Hey desert dog, the best way to keep your MPG up as well as HP/TQ is through a great tune. Making use of every bit of air and fuel that enters your combustion chamber.

Big_Brother

I understand what you're both getting at, I was trying to say that "efficient" is not wot acceleration, max efficiency and max power are not the same thing in my opinion.

At a steady 2000 rpm (an arbitrary figure, I'm using for an example) the engine will not be consuming much fuel and moving an almost 2 ton weight at around 80 mph (yes, gearing plays role, just an example here), and the combustion cycle will not be on the verge of detonation with a low load situation such as that. To me this scenario more accurately defines the term efficient.

Accelerating hard with each cylinder on the verge of detonation may be the fastest way to attain speed, but I'd hardly call it efficient.

I thought I stated that max piston speed is derived from maximum pressure, without detonation.

JamesO WS6

I'm not saying your wrong, just saying your measuring LOAD on an engine. He wanted efficiency of an engine which is measured in VE and RPM is not a factor. B/s the scenario you are describing is the best way to get better gas mileage, not measure efficiency.

BTW, detonation can happen at idle. Not just WOT, the severity of it is the variable.

hondo

iTrader: (2)

heres some great reading on the subject
j-rods timing thread

TXNC5

What measure of 'efficiency' are you talking/concerned about? i.e. How well the engine uses air (BSAC), fuel (BSFC), how well it converts the combustion process into power, etc.? There are different measures of efficiency.

The engine is not necessarily 'more efficient' with a leaner AFR, but it is certainly using less fuel per part of air. This is possible to do when cruising when there is low load on the engine. Fuel injected cars can run much leaner than 14.7:1 with no problems as long as the load remains low. You will have to keep adding timing to allow sufficient time to burn the mixture, but your MPG will definitely increase. If you have a tuning tool, just try to see how lean you can make your car at idle. You will be amazed. Don't worry about melting a piston. It is pretty hard to do with no load.

Desert Dog

What I mean by efficiency...

I have been reading again...

I have been learning a lot about the GM electronic control systems, and I picked this blurb up that the PCM will, if conditions are right, adjust to just "see" where and what other sensors are doing when the ping happens, and back off slightly...

This set off all kinds of bells in my head.

Mike454SS

iTrader: (4)

You are absolutely 100% wrong and need to learn what the definition of VE actually is. If you put a turbo onto an engine, the VE actually goes down slightly because of the restriction in the exhaust.

As for NA engines, Pro-Stock and F1 cars are 2 examples of engines we see all the time that are over 100%...by a lot.

More power per displacement isn't always higher VE. VE is essentially the mass of air and fuel in the chamber when the intake stroke has finished and the valve has closed, compared to the mass of air and fuel that would fill a cylinder of the same volume as the total swept volume (don't include the chamber) of that cylinder in the engine at the same pressure that the intake manifold is at during that specific time that you are chosing to measure VE. Thats why a turbo on the same engine isn't higher VE, and is in fact slightly lower. There's more air/fuel mass in the cylinder, but there's higher pressure in the intake manifold, and the cylinder isn't getting as high a % of it out of the chamber as it would without a turbo because of the fact that the turbine poses a restriction and thus on the next cycle of that cylinder, there are old spent exhaust gases still in there taking up room preventing the engine from inhaling as high a % of air/fuel from the intake manifold (based on intake pressure) as it would have without the turbine there.

It can be an extremely difficult concept to grasp and reading my expalanation of it might not explain it to you as it's difficult for me personally to word it.

Mike454SS

iTrader: (4)

And I'm not entirely sure if it's the mass of air and fuel, or just air. I would think on a carbureted engine it'd be air and fuel mixed as thats whats in the entire intake manifold, but on an injected engine it'd be just the mass of the air.

Hi-Po

iTrader: (4)

DEF- Volumetric Effficiency indicates the total amount of air/fuel mixture each cylinder takes in during the intake stroke. Overall VE can be calculated by applying the formula: VE= 2 x mass airflow rate/ air density x swept volume x rpm. High performance engine builders improve VE by optimizing exhaust scavenging and by taking advantage of inertia.

Big_Brother

Well the title of the thread is Ping, detonation vs engine load, GenIII motors

I also stated that I gave a brief explanation of detonation, I didn't really feel the need to point out that detonation could technically happen at idle as it is a highly unlikely scenario, unless Cooter is tuning the engine.
I don't have the time to write a thesis on the subject.

JamesO WS6

Mike454SS, well I brushed up on the subject. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE). If a turbo/supercharger puts in more air then the displacement of the engine, that alone supports my statement. So with that being said a supercharger or turbo WOULD in fact give a higher VE. Were my percentages excatly correct no, does that really matter? I dont agree that F1 cars would be a good example b.c. of their supperior engines and design.

Big_Brother you are excatly correct about the detonation, i just added my .02c. As for the engine efficiency I beleive we are on two different pages and are not speaking about the same thing.

I would like to understand better why an internal combustion engine operates most efficiently at the verge of detonation.
I didn't beleive that your scenario addressed the question at hand.

technical

Not yet it isn't.

Engines perform best as they near *pre-ignition*, not detonation. There is a huge difference.

SideStep

DanO

First off, please learn about what you are saying before you call someone wrong. James O was far closer to the correct answer than you my friend. If a turbo had a lower VE there would be ABSOLUTELY NO REASON TO HAVE IT! The WHOLE POINT to forced induction is the increase in VE!!!!!! Any time you increase the pressure in the intake manifold of an engine you will be increasing VE. Yes, turbos and superchargers get well over 100% ve but how much over depends on a variety of factors.

And the best NA engines only get slightly over 100% (not by alot) due to tuning the intake and exhaust runner lengths to match the pressure pulses which allows the inertia of the air to increase the cylinder filling.

And if all other factors were equal a higher VE with the same AFR would DEFINATELY produce more power. More air + MORE FUEL = more power.

EdmontonSS

Volumetric efficiency is the measurement of how close the actual volumetric flow rate is to the theoretical volumetric flow rate. If you bolt a turbo onto the engine the theroetical flow rate will increase due to positive manifold pressure, however the engine's actual flow rate will not increase as much as you are now trying to push a larger amount (mass) of air through the same size ports and valves as well as the increased amount of exhaust resriction. Turbocharging may create a more efficient engine but it will not make an engine more volumetrically efficient. BTW, this is the lamest thread in this section... For starters, I want a reliable source that states that an engine runs most efficiently at the verge of detonation (horsepower or fuel consumption rise). That statement is totally false as a blanket statement, AFIK. It sounds similar to the crap about "advance the timing until it pings, then back it off a few degrees for best power". If that uneducated hick tale is what this thread is based on, the thread should get removed from this section.

DanO

Like i told the previous gentlemen, please read up on the subject prior to posting false information. I would like to keep people informed and not have them belive those statements as fact.

"Idealy, a mass of air equal to the density of atmospheric air time the displacement volume of the cylinder should be ingested for each cycle" (Engineering fundamentals of the internal combustion engine, Plukrabek Pg. 69)
VE is based off of displacement volume of the engine and air density evaluated at atmospheric conditions outside the engine

This means the Physical built in piston bore and stroke and atmosphic conditions are the basis for all VE measurements.

Adrenaline_Z

I think what he (EdmontonSS) is trying to state is:

Motor 'A' can fill a cylinder to 85% naturally aspirated.

Motor 'B' can fill a cylinder to 105% with boost @ 15 PSI.

As a relative figure, the 15 PSI boost required to make the 20% increase
is not efficient in comparison.

Nonetheless, torque has risen across the board, therefore VE has improved.

P Mack

iTrader: (6)

So which is it, mass or volume? I don't like that definition because he's saying mass and you're talking about volumetric efficiency. I guess the real question you guys are arguing about is whether ve is the percentage of volume that enters the cylinder at standard atmospheric conditions or at intake manifold actual conditions. (BTW, our VE tables are set up the second way) A turbocharged engine may fill the cylinder less efficiently volume wise, but since the air is more dense it fills it with a greater mass of air.