lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### How Does An Engine Rev Up?

All this time, I have been thinking in steady-state or per cycle.  I have no idea how an engine ends up with a higher RPM.

In a steady state condition, most everything is in an equilibrium with the friction forces keeping it from going on forever.

When I'm driving, to go faster I hit the gas. Duh, but, what is happening in the engine? For a brief moment, there is a larger fuel air ratio, so anything extra will not react with oxygen. Does is somehow participate in increasing the engine's pressure? I donno. Whatever happens, somehow there is a higher pressure produced that foces the piston around faster and that draws more air into the combustion such that the fuel-air ratio will become restored.

Briefly, unless your foot is still on the gas pedal or throttle.

What happened???    I'm confused.

I can understand how a lack of fuel makes the engine slow as friction takes over. I figure the fuel-air ratio becomes too large to sustain pressures required to spin the sucker around. Either was, we're back to square one: Equilibrium.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

DfwRevolution
Posts: 8859
Joined: Sat Jan 09, 2010 7:31 pm

### RE: How Does An Engine Rev Up?

 Quoting Lehpron (Thread starter):For a brief moment, there is a larger fuel air ratio, so anything extra will not react with oxygen. Does is somehow participate in increasing the engine's pressure?

The average fuel air ratio for most engines is something like 14:1, in other words, if you open-up the throttel a bit, there will be pleanty of air to burn the extra fuel. A larger combustion drives the piston down with more force, which turns the crankshaft faster, which causes all the pistons to oscilate with greater speed.

This increases the number of combustions per second.The speed at which the pistons go up and down controls the speed at which the crankshaft output spins.

Kinda hard to articluate if you know what I mean?
I have a three post per topic limit. You're welcome to have the last word.

cptkrell
Posts: 3186
Joined: Sun Sep 23, 2001 10:50 pm

### RE: How Does An Engine Rev Up?

Layman's experiment 101.

More ignited fuel/air mixture equals more burn. Don't try this at home inside: squirt a little more charcoal lighter fluid (or gasoline if you want a real eyebrow-singing result) on your barbecue after it is lit and burning. What happens? More fuel and more air equals more "boom"! Burns more quickly. Contain this experiment in a chamber (cylinder) with one end of the chamber in a moveable configuration (piston) and the piston will move more quickly opposite the more quickly burning (exploding) fuel/air mixture.

So, increasing the fuel supply while simultaneously increasing the air supply will result in an increase of combustion, and in the case of normally-aspirated internal combustion piston engines, will result in the piston increasing its' force, automatically resulting higher RPMs with additional throttle. This effect can, of course, be enhanced by forcing more air into the cylinder via turbo or supercharging.

Just remember more air and more fuel equals more boom.

DfwRev is correct when referring to fuel/air ratios, or, "stoichiometric" mixtures. This is the ideal fuel/air mixture where all the fuel and all the air will be completely used in the contained combustion cycle. But, remember different fuels have different ratios; the stoichiometric ratio of "most" non-oygenated gasolines is 14.6 - 14.7 : 1, whereas, for example, ethanol is approx 9.0 : 1.

Regards...Jack
all best; jack

MrChips
Posts: 933
Joined: Wed Mar 30, 2005 2:56 pm

### RE: How Does An Engine Rev Up?

Everyone's got it pretty close so far, but it could be said a lot simpler.

The combustion of air and gasoline produces a fixed increase in pressure for a given fuel-air ratio. With the engine idling, the intake manifold pressure is quite low...could be as low as 2 pounds per square inch. With a fixed increase in pressure, there is a relatively small amount of pressure generated by combustion.

If you hit the accelerator pedal, you open up the throttle body, allowing the intake manifold pressure to increase. The fuel injection system compensates for more air by adding more fuel. All of a sudden, at full throttle, intake manifold pressure is almost atmospheric (in a normally asipirated engine). Therefore, more air + fuel enters the engine, which in turn generates more force per combustion cycle, driving the piston down faster and with more force, until the engine reaches equilibrium.

Something to remember before you try this on Mom's Windstar - equilibrium speed for an engine with no load on it is almost always well above the engine's ability to remain intact.

With regard to stoichiometric ratios and the like, the reason ethanol doesn't need as much air to burn is because of that wonderful -OH group attached to the end of the molecule. The more oxygen the fuel has attached to it (like nitromethane), the less air is needed achieve a stoichiometric ratio.

EDIT: I guess it isnt so simple after all...

[Edited 2005-05-31 08:42:15]
Time...to un-pimp...ze auto!

Kay
Posts: 1797
Joined: Fri Mar 01, 2002 3:41 pm

### RE: How Does An Engine Rev Up?

I really racked my brain for that one when I was in college and wondering why I wasn't doing a Bachelor in Mechanical Engineering instead of Computer & Communications Eng'g.

The reason was clearly explained above and I won't repeat it, except for an additional comment to MrChips' explanation:

 Quoting MrChips (Reply 3):equilibrium speed for an engine with no load on it is almost always well above the engine's ability to remain intact.

If you have an engine on the bench without the intake butterfly/air filter mount, which means that atmospheric pressure is reaching well inside the manifold, and you start up that engine, it will rev up straight to the red line/limiter instantly. That's right, the butterfly's main role is to kill the intake pressure to sub-atmospheric levels. Makes you understand why an engine consumes so little fuel after all (because, after all, when half a bottle of fuel can keep a one-ton vehicle going around for so long, it is little).

Kay

sacflyer
Posts: 364
Joined: Tue Jan 13, 2004 8:19 am

### RE: How Does An Engine Rev Up?

Speaking of concepts that took a long time to visualize...

When I was in high school, I was fascinated by jet engines and would read every book that I could find on the subject. However, I took me a long time to figure out how the force of combustion was directed out the tailpipe. It always seemed to me that the force would go two ways, out the tailpipe and back into the compressor (which obviously happens during a compressor stall.) But, I couldn't grasp the actual mechanism or concept that kept the exhaust gasses flowing in one direction.
I'm just happy that RR ratings can't be in negative numbers!

Euclid
Posts: 324
Joined: Fri Apr 22, 2005 3:42 am

### RE: How Does An Engine Rev Up?

You've all got it pretty much covered for what happens in a petrol engine, but what about diesels?

A diesel engine has no throttle valve. At idle or at full revs, there is always an unrestricted flow of air going into the engine, wich means that there is always an oversupply of oxygen in the combustion chamber. What stops this engine from running away?

The answer lies in something that a petrol engine does not have, called an injector pump. This pump will send a very carefully metered amount of diesel to the injector, one to each cylinder, so the reaction in the combustion chamber is very carefully controlled.

Stepping on the accelerator will open a throttle body inside the injector pump, allowing the injector pump to send a bit more diesel to each injector, thereby causing a bit of a bigger reaction in the combustion chamber and a rise in engine revolutions.

Interestingly, it is not possible to overspeed a diesel engine by just pushing the accelerator pedal to the floor and keeping it there. Two springloaded weights in the injector pump that turns with the pump and that is connected to the throttle body will be forced apart by centrifugal force, thereby pulling the throttle body closed regardless of what you are doing to the accelerator pedal. The engine will reach a certain amount of revolutions and just stay there.

Of course, you will still overspeed the engine if you are driving at 120 km/h and then jam the gearbox into first and let go of the clutch. Doing this with a petrol or diesel engine will pretty much leave a trail of engine parts on the road behind you.

Regards

Euclid

lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### RE: How Does An Engine Rev Up?

 Quoting DfwRevolution (Reply 1):The average fuel air ratio for most engines is something like 14:1, in other words, if you open-up the throttel a bit, there will be pleanty of air to burn the extra fuel.

Plenty from where? Isn't it best to have a ratio of fuel and burn to be just right so that all the fuel burns only with all available oxygen in the air, such that you don't have unburned fuel or extra oxygen? Or is there a cupter command in the car that senses the gas pedal, then initiates an increase in air at the next cycle (which may be within 1/1000th second or less) then fuel, then you go faster? And for every new cycle, it pumps in more air as long as you want to go faster? How does the cycle pump the air in? Hell, would the delta pressure (from in the engine to ambient) drop as the engines goes faster, that your highest accleration will be at zero, that you eventually reach an RPM maz that heats up your engine, etc. I think I get internal combustion engine accelerations now.

 Quoting Cptkrell (Reply 2):Just remember more air and more fuel equals more boom.

Agree with that I do.

 Quoting MrChips (Reply 3): With the engine idling, the intake manifold pressure is quite low...could be as low as 2 pounds per square inch.

Wait a minute. I don't know much about cars, are you talking about some sort of intake pump? At first this question was gonna get put in the TechOps, I was asking about jet rev-up, but IC and jet burns are the same in therms of fuel just that in an IC it happens once every fourth and jet is all the time. Even in the jet condition, how would there be an air pump, unless there was a motor to accelerate the compressor then add fuel?

 Quoting Sacflyer (Reply 5):But, I couldn't grasp the actual mechanism or concept that kept the exhaust gasses flowing in one direction.

Me too, but I originally had confusion with rockets. Later I found that the fuel and oxidizer were stored under high pressure as it went into the chamber, which kept the thrust from going back up. See with rockets it make sense, the throttle will control the fuel and oxidzer to maintain a fuel-air ratio, 4-cycle and turbine engines are not like that, hence my confusion.

 Quoting Euclid (Reply 6):wich means that there is always an oversupply of oxygen in the combustion chamber. What stops this engine from running away?

Lack of fuel to burn? Just like the charcoal example from above, eh? Isn't the deal with diesels that ther is no igniter, that ignition is self-inflicted via compression, i.e. autoignite?

Okay people, I think ya'll helped me figure it out, but please confirm that for me. If you can explain how a gas turbine gets faster that would be great.

[Edited 2005-06-01 09:17:25]

[Edited 2005-06-01 09:20:13]
The meaning of life is curiosity; we were put on this planet to explore opportunities.

FDXmech
Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

### RE: How Does An Engine Rev Up?

 Quoting Lehpron (Reply 7):Plenty from where?

Plenty from the throttle plate opening allowing more air in.
You're only as good as your last departure.

Fokker Lover
Posts: 523
Joined: Fri May 10, 2002 10:05 am

### RE: How Does An Engine Rev Up?

I think you may be over analyzing it a bit. Remember there is a difference between making the engine run, and making it run really efficiently.
They ran a hundred years ago without computers and super contolling electronics. They ran and produced power, they just didn't run as efficiently as todays.
In the above post you asked about some sort of an intake pump. To answer that I would say that you have to think of the the piston as a multi-purpose tool that has a different function for each stroke.
The piston is the a vacuum pump during the intake stroke. It creates the suction that lowers the manifold pressure.
On the way up it acts as an air compressor. Hence the name compression stroke. Boom, after ignition it acts as a battering ram that has the sole purpose of driving the crankshaft halfway around a circle. That's the power stroke. Then on the way back up it is just a big wall that pushes the exhaust out and gets ready to start again.
If you look at the first single cylinder engines, they had a heavy fly wheel to keep them moving during the 3 strokes that weren't producing power. Today we have multi-cylinders so that when one is sucking air in, another is on the power stroke driving the others. That's why we aren't concerned with inertia to keep everything moving. there will be at least one cylinder on the power stroke at all times.
For some reason I get the feeling that you already knew all of that, but I'm practicing to be a high school teacher.
10,000 years ago we would have eaten you. Today, we drag you along and allow you to pollute the gene pool.

Euclid
Posts: 324
Joined: Fri Apr 22, 2005 3:42 am

### RE: How Does An Engine Rev Up?

 Quoting Lehpron (Reply 7):Lack of fuel to burn? Just like the charcoal example from above, eh? Isn't the deal with diesels that ther is no igniter, that ignition is self-inflicted via compression, i.e. autoignite?

Yes, exactly. I may have been a bit vague in my description, but because only a very small amount of diesel is injected into the combustion chamber, that is controlled by the injector pump, the engine cannot run away.

Ignition in a diesel engine takes place through what may be described as spontaneous combustion.

In a petrol engine, the air and fuel mixture enters the combustion chamber through the intake valve. In a diesel engine, only air is drawn into the combustion chamber. The rising piston then compresses this amount of air into a very small space, causing it to heat up tremendously. A little before the piston reaches top dead center, the diesel is injected into the combustion chamber as a very fine mist. This warm compressed air will cause these very fine droplets of diesel to ignite spontaneously, the resulting "explosion" thus forcing the piston down again. This process requires no spark plugs or electrical power from the vehicle's ignition system, and in some older diesel vehicles that was not fitted with an electrical fuel cut-off switch the engine would just keep running even after the ignition was switched off.

You had to kill these engines with a hand operated lever that would be connected via a cable to a manual fuel cut-off valve mounted on the injector pump. This would just kill the fuel flow to the pump, stopping the engine.

Some trucks did not even have this. On trucks fitted with exhaust brakes you had to step on the button that would activate it, and the build-up of back pressure in the exhaust system would then stop the engine.

Fortunately, someone invented the fuel cut-off solenoid, wich is basically just an electrically operated fuel cut-off valve. Switching on the ignition would energise this solenoid, opening up the fuel line, and switching off the ignition would cause the solenoid to block off the fuel line, killing the engine. Fortunately this switch requires very little power to operate.

As an example, I was driving home one day with my diesel powered pick-up when the alternator failed. This was just before dark, so the lights quickly drained the battery. Eventually not even the radio would work anymore, yet this fuel cut-off solenoid remained open, and I made it home no problem, although in darkness. Fortunately most of the driving was in well lighted city streets.

This of course is not relevant to today's modern diesel vehicles, where everything is controlled by electronics. A failed alternator or computer box will leave you stranded by the side of the road regardless of wether the vehicle is petrol or diesel powered.

Regards

Euclid

lehpron
Posts: 6846
Joined: Tue Jul 10, 2001 3:42 am

### RE: How Does An Engine Rev Up?

 Quoting Fokker Lover (Reply 9):To answer that I would say that you have to think of the the piston as a multi-purpose tool that has a different function for each stroke. The piston is the a vacuum pump during the intake stroke.

If the volume within the cylinder from top dead center to bottom dead center doesn't change, how does varying the throttle plate opening change the amount of air entering the system?

In my mind, all that valve is doing is varying the pressure of the in flow, that unless the opening can vary the density of the air, I do not see how there can be more oxygen when the valve opens all the way. In a small opening, the air must move quickly to fill the space left by the piston moving down; whereas a large opening can have air moving slower into the space.

It's not that I make problems complex, just that if they are too simple, I end up finding new variables or reasons when/where the simplicity cannot work. If someone else was complex in answering, then I will have more options or the chances are greater I find what I'm looking for.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

MrChips
Posts: 933
Joined: Wed Mar 30, 2005 2:56 pm

### RE: How Does An Engine Rev Up?

 Quoting Lehpron (Reply 7):are you talking about some sort of intake pump?

Fokker Lover nailed it - air does not rush into the cylinder on its own, rather, the intake stroke pulls air in.

Think of your engine as a vacuum cleaner - it constantly pulls air into itself. Now place your hand over the end of the hose. That is what the throttle valve does...it prevents the engine from sucking too much air in. Now let us recall the basic principles of a four cycle engine.

Intake stroke - Piston starts at top dead center (TDC), intake valve is open, exhaust valve is closed. The piston moves down in the cylinder, drawing air-fuel mixture in. At or near bottom dead center (BDC), the intake valve closes.

Compression stoke - Piston is at BDC, and both valves are closed, forming a nearly airtight chamber. Piston moves up to TDC, compressing the combustion charge in the process.

Power stroke - With both valves closed, spark plug fires, igniting the charge and causing deflagration (rapid controlled combustion, but NOT AN EXPLOSION!!!). The increase in pressure drives the piston downward with considerable force.

Exhaust stroke At or near BDC, the exhaust valve opens, causing the combustion by-products to rush out, aided by the upward movement of the piston to TDC.

The volume inside of the cylinder changes considerably from TDC to BDC - I have a feeling you are looking at it from a displacement point of view (the net motion is zero for the cycle, therefore, no volume change occurs). Take that view and throw it out the window, because it will only lead to more confusion.

 Quoting Lehpron (Reply 11):In my mind, all that valve is doing is varying the pressure of the in flow, that unless the opening can vary the density of the air

Remember, density and pressure essentially the same.
Time...to un-pimp...ze auto!

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