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Old 11th May 2005, 17:02   #1
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Default are spark plugs really required all the time???????

we all know that engines require air-fuel mixture to run .and for this fuel to burn its the spark plugs that provide the initial spark.

so once the fuel ignites,it keeps on burning till there is a constant supply of fuel .

so what i want to know is,do we really need the plugs after this initial spark.
what if we remove the plugs when the engine is running???????

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Old 11th May 2005, 17:16   #2
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Quote:
so once the fuel ignites,it keeps on burning till there is a constant supply of fuel .
No, you misunderstood.

Spark plugs don't ONLY provide the initial spark, they continue all the time. If there was no spark plug, the fuel would never burn. Each time air-fuel enters the chamber, it is compressed and then the spark plug ignites it.
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Old 11th May 2005, 17:22   #3
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I was about to put up the same message but gordon you caught me napping.

Try pulling out the spark plug when running a 2 wheeler and it would stop in no time,
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Old 11th May 2005, 17:25   #4
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as long as there is a constant fuel supply,why do u need another spark .

even if fuel does not come in a constant spray,once the initial supply is exhausted there should be a lag in performance till the next supply starts.

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Old 11th May 2005, 17:27   #5
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Hmmm while we are on the topic of spark plugs what i wanted to know is, does it make any diffrence if you use prformance plugs on an unmodiffied engine and if yes what are the ones available and for how much are they????
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Old 11th May 2005, 17:30   #6
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SpeedSatya,

Constant air/fuel supply is provided, but how will the mixture burn after the initial ignition?!??!

To make it simple, consider a closed chamber with a spark plug. Air/fuel mixture is supplied to it through valves and the valves are closed. The mixture is then compressed and ignited using the spark plug. The resultant exhaust gases are then let out through another set of valves, which is then closed. The same process is repeated. Again air/fuel mixture is let in, compressed, and then ignited using spark plugs [if not, how will the mixture burn?!??!]. And again the resultant exhaust gases are let out.
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Old 11th May 2005, 17:31   #7
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satya -- i might be wrong but this is what i feel

the spark that is delivered might be enough for the air fuel mixture that enters at one cycle to burn, so every cycle when fuel enters there needs a spark to burn it, cause once the fuel is burnt there is no more cumbustion factor to assit combustion
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Old 11th May 2005, 17:33   #8
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SS - the engine runs in 4 cycles. Eventhough it may seem to take place instantaneously, the cycles do take place.

Cycle 1 - the charge is fed into the cylinder
cycle 2 - the charge is compressed and ignited
cycle 3 - the charge burns and pushes the piston down (power stroke)
cycle 4 - the burnt charge is expelled

See there is no constant fuel supply here. fuel is being supplied only at the first stroke. only after the fuel is completely burnt off and expelled, fresh charge is taken in to the combustion chamber.

So the fuel has to be ignited everytime as it gets compressed get it?

Of course this is assuming that its a petrol engine. Petrol has to be ignited by a spark. In a diesel engine the fuel is compressed to upto 1000bars and injected into the chamber. When it gets compressed, the rise in temperatue (given by Charle's and boyle's law) is enough to ignite the charge.
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Old 11th May 2005, 17:39   #9
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Actually it is possible to run an engine without spark plugs, you just need the right kind of compression. Look at diesel engines, they don't use spark plugs, they just rely on high compression to ignite the fuel.

Quote:
Petrol has to be ignited by a spark
Then how do you explain pre-inginition or knocking. This is caused when the fuel ignites even before the spark plugs light up (due to higher compression than what the fuel was designed for).

It's not impossible but, the fact is that it's easier to use a spark plug. I'm sure if companies were challenged to come up with a spark plugless petrol engine they might be able to.

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Last edited by Shan2nu : 11th May 2005 at 17:41.
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Old 11th May 2005, 17:39   #10
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Harrie explained it with more detail. For more information, consider the following link:

HowStuffWorks "How Car Engines Work"
http://auto.howstuffworks.com/engine1.htm
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Old 11th May 2005, 17:40   #11
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Quote:
Originally Posted by Harrie

See there is no constant fuel supply here. fuel is being supplied only at the first stroke. only after the fuel is completely burnt off and expelled, fresh charge is taken in to the combustion chamber.


.

ok ,so wont there be lag in performance as when the previous supply is burnt ,the fresh supply will take some time.

Quote:
Originally Posted by Harrie

So the fuel has to be ignited everytime as it gets compressed get it?


.
so do u mean to say that its only at this stage that a spark is required.
if this is true,then also u dont require constant sparkingall the time
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Old 11th May 2005, 17:41   #12
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Well, now i got a question for you guys... if its easier to ignite fuel with sparkplugs... then why not have diesel engines with spark plugs?? Whats the reason for not using spark plugs in diesel engines??
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Old 11th May 2005, 17:47   #13
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Quote:
ok ,so wont there be lag in performance as when the previous supply is burnt ,the fresh supply will take some time.
It's not that slow. An engine usually idles at 800 rpm. And it's not that all pistons move in the same direction at the same time.

Quote:
so do u mean to say that its only at this stage that a spark is required.
Yup thats true, except in the NHC which has 2 spark plugs which vary their spark timing depending on the engine rpm. But, even here, both plugs ignite within fractions of a sec. It's so quick that you maynot even notice the diff if you could see inside the engine.

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Old 11th May 2005, 17:50   #14
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Quote:
Well, now i got a question for you guys... if its easier to ignite fuel with sparkplugs... then why not have diesel engines with spark plugs?? Whats the reason for not using spark plugs in diesel engines??
A gasoline engine takes in a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel.

The higher the compression ratio, the more power is generated.

If a diesel engine was to work like a petrol engine the kind of power out put from it would be even more pathetic. We'd have Accent crdis' producing 45-50 bhp, maybe even less.

Shan2nu

Last edited by Shan2nu : 11th May 2005 at 18:06.
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Old 11th May 2005, 17:55   #15
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Petrol is more volatile than diesel. Also the physical properties of petrol cannot be arrived at accurately due to the change in composition, carbon structures, etc etc., But the general property is like this - compress petrol to say 50 bar, it will explode! simple as that.

That is why the fuel injection pump compresses it to about 10-15 bars only. If you compress it any further it MIGHT explode. This also explains why high compression engines need better quality fuel. Poor quality fuel might start burning at a low compression itself.

So in order to control combustion, what we do is compress petrol to its maximum safe limit and then ignite it by a spark. Of course you can ignite it by just compressing alone but then the number of variable factors involved are many.

1. the composistion of the fuel
2. the composistion of air
3. the temperature of fuel
4. the temperature of air
5. the temperature of combustion chamber bla bla

So to make things simpler and fool proof, we use a spark plug to ignite the fuel exactly when required.

But remember that there are 4 cylinders with 1 spark plug in each ( 2 in pulsar DTSI and the NHC???) so depending on the firing order one or the other spark plug will be sparking always. This is necessary or else you will feel the lag in performance. The cylinders operate in such a way that atleast one cylinder produces power at any point of time.

It is practically impossible to produce a sparkplugless engine unless we are able to use a fuel that has stable physical properties over a wide variety of conditions unlike petrol.
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