What happens when you press the gas pedal?

[Z064life]

Hi,

I read this article ( http://ezinearticles.com/?What-Happens-When-You-Hi... about what happens under the covers when you press brake. This isn't the most indepth or accurate explanation, but it will do.

However, what happens when you press the accelerator? I know that there is a combustion cycle which is kicked off but I don't know how the accelerator pedal's pressed state kicks all of that off.

Thanks

[blueg33]

Put very simply, the accelerator is connected to a valve that opens more as the pedal is pressed. this valve lets more fuel into the engine, more fuel = more power = more speed.

Modern cars have the pedal connected to the valve via a computer, older more basic cars use a cable that is moved mechanically by the pedal and thus opens the valve mechanically.

You dont need the pedal to start the engine as the car is usually set up to draw enough fuel to idle without input on the accelerator.

The engine fires because a spark ignites the fuel, each spark is fired by the distributor which is connected to the crankshaft. The distributor is a sort of rotary switch that fires sparks in turn. Modern cars use a computer to do this. The electricity for the spark comes from the coil, which is basically a transformer powered by the battery. The battery is charged by the alternator when the engine is running.

Edited by blueg33 on Saturday 16th October 15:17

[DaveL485]

http://lmgtfy.com/?q=how+a+car+engine+works

[Pumaracing]

blueg33 said:

Put very simply, the accelerator is connected to a valve that opens more as the pedal is pressed. this valve lets more fuel into the engine, more fuel = more power = more speed.

Hilarious. The blind leading the blind. I suppose it could almost be a desription of how a diesel engine works but as to how a petrol engine one works which is no doubt the question being asked it's pure gibberish.

[agent006]

Pumaracing said:

blueg33 said:

Put very simply, the accelerator is connected to a valve that opens more as the pedal is pressed. this valve lets more fuel into the engine, more fuel = more power = more speed.

Hilarious. The blind leading the blind. I suppose it could almost be a desription of how a diesel engine works but as to how a petrol engine one works which is no doubt the question being asked it's pure gibberish.

How about you offer a better one then?

[blueg33]

Pumaracing said:

blueg33 said:

Put very simply, the accelerator is connected to a valve that opens more as the pedal is pressed. this valve lets more fuel into the engine, more fuel = more power = more speed.

Hilarious. The blind leading the blind. I suppose it could almost be a desription of how a diesel engine works but as to how a petrol engine one works which is no doubt the question being asked it's pure gibberish.

I stated that it was very simplistic. So in laymans terms and simplistically you tell me where I am wrong and describe it yourself. And of course it could also be a diesel engine because the process is the same! It could also be a gas turbine engine.

I think tonight must be Pistonheads Fatuous Comments night!

Edited by blueg33 on Monday 18th October 22:31

[Iain328]

Pressing the gas pedal increases the airflow into the engine. That in turn draws more fuel in (or in the case of electronically controlled injection causes more fuel to be pumped in).

The throttle pedal acts on a butterfly valve (or a series of valves) in the inlet tract which control the flow of air, not fuel.

[Silent1]

The throttle butterfly acts as a restriction in the engine and by decreasing the amount of air you decrease the revs.

It you can get hold of the current shell v-power club magazine called v-zine there's a brilliant 4 page article on it and why things like direct injection and the new fist engine are very impressive

[blueg33]

Iain328 said:

Pressing the gas pedal increases the airflow into the engine. That in turn draws more fuel in (or in the case of electronically controlled injection causes more fuel to be pumped in).

The throttle pedal acts on a butterfly valve (or a series of valves) in the inlet tract which control the flow of air, not fuel.

You contradict yourself in the last sentence. Pressing the gas pedal does let more fuel in, it happens to do it on a carb engine by using the airflow to draw the fuel in, a valve in still opened and that causes more fuel to enter the cylinder drawn in by the air. In my very simple explanation i decided to leave fuel air mixture out of it.

In an injected engine? Each injector is a valve and doesn't pressing the throttle pedal increase both the air available and the injection of fuel?

[Iain328]

blueg33 said:

Iain328 said:

Pressing the gas pedal increases the airflow into the engine. That in turn draws more fuel in (or in the case of electronically controlled injection causes more fuel to be pumped in).

The throttle pedal acts on a butterfly valve (or a series of valves) in the inlet tract which control the flow of air, not fuel.

You contradict yourself in the last sentence. Pressing the gas pedal does let more fuel in, it happens to do it on a carb engine by using the airflow to draw the fuel in, a valve in still opened and that causes more fuel to enter the cylinder drawn in by the air. In my very simple explanation i decided to leave fuel air mixture out of it.

In an injected engine? Each injector is a valve and doesn't pressing the throttle pedal increase both the air available and the injection of fuel?

Usually best to stop digging when you find yourself in a hole of your own construction

You are splitting hairs (and not very well!).

Fundamentally the accelerator pedal on any engine controls the airflow to the engine and the airflow in turn controls the flow/draw of fuel. Trying to work the thing the other way (i.e. metering in precise amounts of air to a given quantity of fuel would be impractical/impossible)

In modern engines both the throttle body and the fuel injection may be electronically controlled but in principle it is still the airflow that governs the fuel draw as opposed to the amount of fuel you pump in governing the amount of air delivered into the engine.

[blueg33]

Iain328 said:

blueg33 said:

Iain328 said:

Pressing the gas pedal increases the airflow into the engine. That in turn draws more fuel in (or in the case of electronically controlled injection causes more fuel to be pumped in).

The throttle pedal acts on a butterfly valve (or a series of valves) in the inlet tract which control the flow of air, not fuel.

You contradict yourself in the last sentence. Pressing the gas pedal does let more fuel in, it happens to do it on a carb engine by using the airflow to draw the fuel in, a valve in still opened and that causes more fuel to enter the cylinder drawn in by the air. In my very simple explanation i decided to leave fuel air mixture out of it.

In an injected engine? Each injector is a valve and doesn't pressing the throttle pedal increase both the air available and the injection of fuel?

Usually best to stop digging when you find yourself in a hole of your own construction

You are splitting hairs (and not very well!).

Fundamentally the accelerator pedal on any engine controls the airflow to the engine and the airflow in turn controls the flow/draw of fuel. Trying to work the thing the other way (i.e. metering in precise amounts of air to a given quantity of fuel would be impractical/impossible)

In modern engines both the throttle body and the fuel injection may be electronically controlled but in principle it is still the airflow that governs the fuel draw as opposed to the amount of fuel you pump in governing the amount of air delivered into the engine.

I admit that I should have referred to air rather than fuel, what I really meant was fuel air mixture. I was trying to keep it simple and decided not to differentiate the fluids that enter the combustion chamber.

[shouldbworking]

Well he could be right - after all if carbs are being used theres the accelerator pump jet (probably using the wrong name) that chucks in a little extra fuel on the accelerator being depressed.

Actually, why is that needed? the float chamber should never run dry even on constant full bore so why does it need it on throttle opening? given it goes in prior to the float valve does it just end up sitting there?

hmm maybe its for startup... answers on a post

[stevesingo]

The accelerator pump is used to take account of the sudden increase in demand for fuel when the throttle is opened quickly. The rapid reduction in inlet manifold pressure causes the fuel to condense in to larger, less combustible droplets, some of which condense on the surfaces of the intake tract causing a lean mixture. The accelerator pump alleviates this.

For the benefit of the OP, a spark ignition four stroke engine output is regulated by the amount of air it consumes. This is because, unlike compression ignition engines using diesel normally, petrol combusts efficiently within a relatively narrow air fuel ratio, around 14.7:1 by mass(although it will combust at any thing between about 7:1 - 20:1 air fuel ratio).

When we try to release small amount energy through combustion of petrol, at idle for example, we can only combust that small amount of petrol with a small amount of air. Therefore it makes sense to meter (through a throttle) the amount of air required to combust that amount of fuel for the idling engine. When we want full power from the engine we allow more air in, through the opening of the throttle, to match the amount of fuel required to produce the power we need.

So, What happens when you put your foot down? You open a valve (throttle) and allow more air in to the engine. The engine management system measures the amount of air and adds the correct amount of fuel for efficient combustion. In the case of a carburettor, (very basically) the throttle valve is opened, but fuel is drawn in to the incoming air stream through a carefully sized hole that is matched to the engine characteristics.

HTH

Steve