OK, I know how an internal combustion engine works - just made an airfix-type working model with my six year old son- but what makes it accelerate? How does squirting more fuel into the cylinder actually increase the firing rate - why doesn't it just make a bigger explosion? Er - does that make sense?
ps www.howstuffworks.com - addictive site!
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My basic understanding is that you press the pedal, the throttles open and the throttle position sensor informs the ecu that you want to accelerate and that the throttles are fully open and supplying more air.
The ECU then, via the distributor module, increases the frequency of the sparks and the fuel injection pulses resulting in greater rpm and as such acceleration.
If it just made a bigger explosion I guess you would have an increase in torque, but no increase in crank shaft rotation resulting in no acceleration.
Thats my guess anyhows :)
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>>why doesn't it just make a bigger explosion?<<
It does, and the bigger explosion accelerates the piston faster than a small one, so the next explosion in the next cylinder happens more quickly... i.e. it accelerates.
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Sorry, PB - your post wasn't there when I started!
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It does make a bigger explosion, and that makes things happen faster. The throttle is so named because it 'throttles' the intake and limits the amount of mixture admitted. Pressing the accelerator is actually releasing the throttle, letting the engine do what it wants to, rather than 'pushing' it to go faster.
HTH
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Before rev limiters at least, it could be claimed that unrestricted acceleration caused the explosion!
Cheers,
Tomo
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Of course in the vastly superior and much simpler diesel engine, it isn't a throttle at all. Push the accelerator, the pump injects more diesel, you get a bigger bang, and the engine runs faster.
Richard
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If diesels are so simple, howcome my lawnmower hasn't got one?
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Probably because they're damn noisy and they're b*****s to start by hand due to the high compression ratio.
Richard
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when I used to play on building sites as a child there was a lever to 'turn off' the compression on dump trucks. You spun the engine/flywheel faster and faster against no resistance, then flipped the lever to reinstate the compression, then the engine started. [this was when the builders carelessly left the starting handle lying around!]
I see no reason why this could not be applied to small cars, bikes and lawnmowers - then you would not need any electrical starting gear which would somewhat offset the increased weight of the engine.
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Changing topic slightly, the bit that I have never seemed to understand is torque vs bhp.
If torque is twisting power what has bhp got to do with things?
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CM
Torque is twisting effort, but you only do work when something is moved against a resistance. So Power is a combination of force (torque) and movement (engine rotation)
BHP= 2 x pi x N x T/33,000
Where N = engine speed in rev/min and T = torque in pounds-feet
(I never got into metric on this, but 1bhp = 0.746kW)
Regards
John S
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Torque is twisting effort, but you only do work when something is moved against a resistance. So Power is a combination of force (torque) and movement (engine rotation) BHP= 2 x pi x N x T/33,000 Where N = engine speed in rev/min and T = torque in pounds-feet
Oh well that makes it all very clear!!!!! ;-}
Bogush said that torque was effective leverage - ok. So what is horsepower?
I am a very simple layman and realise that HP was (probably) originaly a measure of pulling power. i.e. my car has 190 odd bhp which I take to mean that if 190 horses are attached to the back of the car pulling in the opposite direction then none of us will go anywhere.
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Yes, torque is 'leverage', and power is the rate of doing work. Torque isn't power - you only get power when something is moved against a resistance.
Thus the equation gives the relationship between torque, engine speed and power. Power is related to the torque and the distance it moves in a certain time ie the engine speed.
1 horse power was defined in the early days of steam power as the work an average horse can do. That is raise 550 lbs a vertical distance of 1ft in 1 second, or 33,000 lbs, 1 ft in 1hr. We have a load, a distance and a time.
That's the source of the number 33,000 in the equation.
It's not pulling power - you can't apply 190 hp statically. If it's not moving no work is being done. For example tugs have engines of several thousand horse power, but their towing ability is measured as 'tonnes bollard pull', not horse power.
Regards
John S
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johnny
Sorry, replied in a bit of a hurry the other day. I meant to add that perhaps the best way to describe your 190 hp engine is to say the it can do the same work as 190 horses. If there was a factory, say, that used 190 horses on treadmills or something, your car engine could be connected up and replace them. This sort of comparison was very relevant in the early days of steam engines, as you can imagine.
This is 'brake' horse power, by the way, different from the old RAC horsepower used early last century for car taxation. That system was based upon a formula derived fron the engine cylinder bore (B x B x N/2.5, where B =bore dia in inches, N = no of cylinders) so had no relevance to power output. This is the reason for car manes such as Morris '8', Austin '7' etc. The result was car engines with small bores and long strokes to achieve the required capacity whilst reducing taxation to the minimum.
Regards
John S
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Think of torque as effective leverage.
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johnny
It's not exactly a 'bigger bang'. An IC engine works by heating and expanding air in the cylinder thereby creating pressure and moving the piston. The heat is supplied by combusting the fuel. That's why it's called 'internal combustion'. A steam engine, for example is 'external combustion'.
On a petrol, opening the throttle allows more air in, which combined with the correct amount of fuel is heated and expanded. The more air and fuel, the greater the maximum pressure, so the more power.
A diesel has no throttle, but intakes air and varies power by burning more or less fuel. Less fuel = lower temperature = less expansion = less power.
Regards
John S
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So even a so-called 'injection' petrol engine still relies on 'suck' to get the fuel into the cylinder? Injection merely means it has been squirted at low pressure into the inlet manifol?
Unlike a diesel which injects at high pressure straight into the compressed air in the cylinder?
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I don't know the answers to any of your questions but I'd definitely be interested to know.
I'd also be interested to know where this leaves the new direct injection petrol engines like those be introduced by Audi. Are they a sort of half-way house between normal petrol engines and diesels?
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DoddMan
These are like the Misubishi GDI (Gasoline Direct Injection). The normal petrol fuel injection has either single or multi-point injectors in the main inlet manifold or in each cylinders inlet respectively. The direct injection has a higher pressure pump to pump the petrol into the cylinder directly, though i'm not to sure if this is into a pre-combustion chamber such as fitted to older diesel engines. The petrol injection pressure will be far below the pressure used in diesel engines.
Bill
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Long, long ago, when I went to school we were taught that petrol engines are quantatively governed, and diesel engines are qualitatively governed.
This means:
When you change the throttle setting on a petrol engine you allow more or less air and fuel to be sucked in on each induction stroke. The air/fuel ratio is controlled by the carburettor or engine control unit, which has various laws built in to allow for engine speed, external temperature, atmospheric pressure, humidity and anything else the designer includes. The spark timing is also adjusted to maximise power.
On a traditional diesel engine the throttle is always wide open, and the air/fuel ratio is changed, by squirting in more fuel, to get more power - hence the slower acceleration of the old diesels.
Modern common rail diesels have the air and fuel premixed in a high pressure chamber, so I presume both the air/fuel ration and the quantity injected are controlled. Hence the better burning, better power, lower noise levels etc.
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Cliff
Yes, injection refers only to the method of injecting fuel, and air still reaches the cylinder due to the partial vacuum created when the piston descends on the inlet stroke. There is actually no such thing as 'suck'. The partial vacuum is filled by the ambient air pressure. Because that limits potential air flow, turbo- or supercharged engines were developed to increase the air pressure and allow more efficient filling of the cylinder with air.
The basic methods of injection are as follows:
Diesel engines:
Indirect injection uses a second, small, combustion chamber, connected to the main combustion chamber by a passage. The fuel is injected into this small chamber, where it is ignited by the hot air created by the compression stroke. The flame front then travels through the connecting passage into the main chamber. The result is a more controlled and therefore quieter combustion, which is why it was adopted for car engines. The classic design is the Ricardo 'Comet' cylinder head. The downside is that the slower combustion reduces the maximum feasible engine speed. Also, the secondary chanber reduces the options for combustion chamber design as it's more difficult to obtain the necessary high compression ratio of a diesel.
Direct injection does without the small chamber and the fuel is injected into the main combustion chamber. This allows faster, but noisier, combustion, although it does have efficiency benefits. It is used on large diesel engines, and made its way into vans but has only recently been adopted for car engines. Higher fuel pressures and more sophisticated injector systems have been developed to reduce the noise usually associated with DI on smaller engines.
Petrol injection:
Most petrol injection systems to date have used injectors mounted in the inlet manifold, which spray the fuel into the air being drawn into the engine, thus mimicking the effect of a carburettor. Various sensors determine the correct fuel supply, and this makes fuel injection more responsive and efficient than a carburettor. Early systems (eg Bosch K-Jetronic) used continuous fuel spray, but later systems have standardised on pulsed supply, timed with the engine operation.
Direct injection supplies the fuel direct to the combustion chamber, at the appropriate point on the compression stroke as in a diesel engine. It needs higher fuel pressures and vary accurate metering of supply. Ignition is still provided by a spark plug, rather than the compression - ignition system of a diesel.
Regards
John S
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