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Old 10th July 2009, 03:33   #1
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Default Petrol engine FAQ

It's my first post so thought I'd make a contribution with it. Also please suggest some more FAQ's to add to this. (btw not to brag but I do engine based research projects so I know this stuff off the back of my hand)

Petrol Engine FAQ

Will a turbo improve my FE?

Generally no.. What a turbo does is eliminate pumping losses at WOT (And create a positive pumping loop) this improves thermodynamic efficiency and thus BSFC (google it). However it also creates an exhaust back pressure for all throttle positions (even with the wastegate fully open). This reduces thermal efficiency at part throttle. Also with the turbo spooling, more oxygen enters the cylinders and thus more fuel injected (infact as this happens at WOT your ECU runs a rich mixture) leading to more fuel consumption. However carefull smart use of a turbo
can save a bit of fuel though not enough to offset the extra cost.

What to look for in an engine?

In an engine look for good power, torque, refinement and consumption. The advertised FE, bhp and torque for an engine are like pmpo for speakers.. Don't take them too seriously. What matters is the
BSFC, torque and power curves, which are actually pretty tough to find. Thus the best way to compare them is to use them.

Oops I filled deisel..am i screwed?

Petrol engines are known as spark ignition engines (SI). They use a spark plug for ignition. They also run on compression ratios of about 10. The conditions thus generated are insufficient for deisel ignition, thus the engine just sputters and dies. All thats required is to flush all the deisel out and fill up on petrol. If your really unlucky then your injectors clog up and spark plugs foul. These are a little expensive to repair.

What about water in my engine?

Well, this depends on how the water got in. If through the fuel tank on a cold engine, it pretty much just needs to be flushed out. If through the manifold on a hot engine, your stuck with an expensive paperweight. The cold water will flood the engine, somtimes cracking very hot components like exhaust valves. Through the exhaust is the worst way to get water in. This happens often while forging through knee deep water when the driver closes the throttle of a high revving engine. Due to the manifold vacume and valve overlap water gets sucked throught the tailpipe, damaging the expensive catalytic converter and other components on its way.

I want Fast and the Furious style acceleration. Is NOS the way to go?

Firstly theres no way to accelerate with a blur except on the enterprise so keep expectations real. 1 mol of nitrous has about 33% oxygen more than 1 mol of air. Thus if in your 1.5 you squeezed in that much nitrous
and petrol you'd get upto 33% (just like sales) more power (assuming 100% VE). However such a mixture would not ignite let alone give more power. A few ml of nitrous would give you a few more ml of O2 and a
bit more power. On the flipside, this raises peak pressures, which may cause structural damage. So with NOS you get a few extra hp but higher costs and wear. However an expensive advanced system could produce
quite large power increases.

I'm sick of conflicting answers. What are the dangers of redline?

Engines are usually designed with maximum operating speed in mind. Manifolds, valves, springs, etc are chosen to allow redline operation. Unfortunatly friction depends on squares which mushroom after 5
(plus the factor of 1000 squared). Thus high up on the rev band, there is a lot of friction which brings on high BSFC, heat and wear. However the engine is designed with this in mind.So redline all you want but dont cruise at 6000 rpm, no matter how good that free flow sounds.

Im one of those never above 3000 drivers. Is it true I need to push it once in a while for the engine?

Petrol engines run on well, petrol.. which is a hydrocarbon. As we all know HC's burn leaving soot and tar (albeit very little). This along with engine oil collects on spark plugs and walls. This fouls the plugs and raises the compression ratio (which can cause knocking). Thus a nice high rev WOT run flushes out all these deposits, leaving your engine clean. Plus it reminds you just why you bought that "less efficient" petrol car.

Do I really need to run in?

Definitly. All but the most expensive engines are made of parts that are cast and then machined. This method of manufacture has relativly bad tolerances. Thus a new engine has bits of metal that are slightly
out of place. Runnung the engine repeatedly bends these bits till they bend into place or fall off.This is why you find swarf on your first oil change. If the engine is revved, these bits are bent extremly quickly and violently, thus causing more metal around it to also bend or break off.

Is high octane worth it?

No! All cars sold in India are designed for 87 octane. The only reason to migrate to higher octane is knocking.

Last edited by Rehaan : 10th July 2009 at 15:00. Reason: Formatted for easier reading.
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Old 10th July 2009, 11:11   #2
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Welcome to forum mosquitoemurderer.Indeed a very good information you have contirbuted to the forum.
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Old 10th July 2009, 11:23   #3
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Welcome to the forum sir, very useful info you have provided here. So, first post pe sixer huh?
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Old 10th July 2009, 11:39   #4
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Please spell out the abbreviations - like NOS and BSFC. A FAQ will be read by absolute newbies, and they will be left wodnering what those words mean.

A couple of questions on fuel injection systems, spark plugs, etc. too would help.

If you can, add "Q" and "A" at the beginning of the sentences. And make the questions or answers in bold, so that it makes for easier reading.

Nice nick, I thought you were from Kochi. ;-P
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Old 10th July 2009, 12:10   #5
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NOS- Nitrous Oxide
BSFC- Brake specific fuel consumption, in brief: Its the amount of fuel consumed, divided by the power being produced.

Last edited by dadu : 10th July 2009 at 12:14.
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Old 10th July 2009, 12:18   #6
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Welcome to the forum and good information to begin with. As BackseatDriver said, a little bit of formatting would add great deal of value to the content.
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Old 10th July 2009, 12:36   #7
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I thought of better formating just after posting, but i cant find an edit button at the bottom right..must be as I am a newbie and need mod approval for it

I'll also extend the FAQ in a day or so

NOS- Nitrous Oxide Systems (a nitrous company)
though it has become a generic name for nitrous
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Old 12th July 2009, 15:33   #8
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Welcome mosquitoemurder.
A very Good compilation. Would look forward to new entries in the future.
Just by the way, I wonder if "Is NOS the way to go?" would qualify as a frequently asked question.
(your contribution is already crawled by Google. May try searching using any one haeding of your faq.)
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Old 13th July 2009, 08:40   #9
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Greetings mosquitoemurder,
That was a great post. Thanks, it answered a lot of questions.

I have some more questions though:
What exactly is knocking? What causes it?
How does one identify it? What can be done about it?
What are the damages cased if left unchecked?

Sorry for the long list but it's something I'd really appreciate you clarifying.

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Old 13th July 2009, 18:12   #10
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that was some interesting reading.

I would really like to know how you came up with your userid !
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Old 14th July 2009, 13:27   #11
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An informative and interesting read.
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Old 18th July 2009, 23:38   #12
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FAQ part 2

Why is my car less powerfull in CNG mode?

The power an engine produces is directly related to the amount of air it breathes. A measure of this is whats called volumetric efficiency or VE. When a gaseous fuel like CNG is used, a large amount of air is displaced as compared to a liquid fuel. This reduces VE and thus power.

What is knocking?

This is the term automotive engineers fear the most. From pierced pistons to broken crankshafts, knocking can destroy an engine in seconds. Think of fuel as an inflated balloon in the cylinder. Normal combustion (deflagration) can be considered as the balloon knot unfolding, there is a quick controlled pressure rise which is converted to power. Knocking however is akin to the balloon popping; a destructive pressure increase.

So what exactly happens?

What we have in the chamber is an air fuel mixture (AFM). A spark from the plug ignites this AFM and thus flames are formed which 'burn' outwards. The flame boundary or flame front moves with a definate speed. Unburnt gasses in front of the flame front get compressed more and more. If their temperature goes too far up (above SIT or self ignition temperature), the gasses explode and set up standing pressure waves which are the cause of knocking.

So when does this happen?

Any time unburnt gasses are heated beyond SIT. This can be caused due to a high gas temperature after the compression stroke. Another cause is the engine being forced to run too slow (usually too high a gear for car speed). Peak pressure is usually realised about 15degrees of crankshaft revolution after the piston reaches the top. So as the flame front progresses down the cylinder, the piston moves down too. this keeps the unburnt gasses fron being pressurised too quickly and exceeding SIT. When the engine is too slow, so is the piston ... ping ping ping, ping ping.

How is it avoided?

The ECU delays the spark, thus lowering pressures throught. This however changes MEP (mean effective pressure) of the power stroke, leading to lower performance. High octane fuel has a higher SIT and thus can be used.

Which is more dangerous; slow speed knocking or WOT knocking?

WOT knocking as resultant pressures are higher.
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