Boost or More Timing ? Can someone explain for me ?
08 April 2004, 10:17 AM
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Boost or More Timing ? Can someone explain for me ?
Hi all,
I have a very knowledgable chap tuning my car for me, and we have put on a hybrid turbo to my spec. It will allow boost to a max of 2.2bar, but I was planning on running a max of 2, which is still pretty high.
Anyway, we were discussing this but then my mate mentioned that it may be better to keep the boost at 'say' 1.5bar and run more ignition. I understand that keeping the boost pressure lower will reduce power losses through heat, but I don't understand what 'running more ignition' means.
Can anyone help ?
Ta
Matt
I have a very knowledgable chap tuning my car for me, and we have put on a hybrid turbo to my spec. It will allow boost to a max of 2.2bar, but I was planning on running a max of 2, which is still pretty high.
Anyway, we were discussing this but then my mate mentioned that it may be better to keep the boost at 'say' 1.5bar and run more ignition. I understand that keeping the boost pressure lower will reduce power losses through heat, but I don't understand what 'running more ignition' means.
Can anyone help ?
Ta
Matt
08 April 2004, 10:48 AM
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Scooby Regular
The period between the spark firing and the complete combustion of the fuel/air mix is very short - on average only about 2 milliseconds. Ignition of the fuel/air mix must take place sufficiently early for the peak pressure caused by the combustion to occur just as the piston has passed Top Dead Centre, and so is on its way down the cylinder bore. If the ignition occurs a little too early, the piston will be slowed in its upward movement, and if it occurs too late then the piston will already be moving downwards, so reducing the work done on it. If the spark occurs much too early, the ignition pressure wave can ignite the mixture in various parts of the combustion chamber, causing detonation.
If the composition of the mixture were constant (and it isn't!), the elapsed time between ignition and full combustion would remain about the same at all rpm. So, if the ignition advance angle were set at a fixed angle before Top Dead Centre, then, as the engine speed increased, combustion would be shifted further and further into the power stroke. This is because the faster moving piston would be further down the bore by the time combustion actually occurred. To prevent this, the ignition advance must increase as engine speed rises.
In addition to engine speed, the other major factor affecting the amount of advance required is the engine load. At light loads (ie when lean mixtures are used) the speed of combustion is slowed and so more ignition advance is needed.
But unfortunately not only does engine speed and load determine the best timing for the combustion of the mixture, but the following factors are also relevant:
the design and size of the combustion chamber
cam timing, especially in variable valve timed engines
the position of the ignition spark(s) in the chamber
the fuel characteristics
the emissions levels required
engine coolant and intake air temperature
the safety margin required before detonation occurs
The emissions of an engine will be affected by the ignition timing that is used, in addition to the air/fuel ratio. Oxides of nitrogen increase as ignition timing is advanced. Running light-load advances of 40 or more degrees is common, giving good responsiveness off load, but if emissions standards need to be met, this advance may have to be reduced. On the other hand, the emission of carbon monoxide (CO) is affected very little by ignition timing, being much more influenced by the air/fuel ratio. At stoichiometric and lean air/fuel ratios, increasing the ignition timing can reduce specific fuel consumption substantially. Finally, the emissions of hydrocarbons at stoichiometric and rich air/fuel ratios increase with advanced timing, but timing has little influence at very lean air/fuel ratios such as 19:1.
It's impossible to ascertain the best ignition timing by juggling all these interrelating factors on paper. Instead, making real-time dyno changes to the ignition timing while using an exhaust gas analyser or air/fuel ratio meter and a means of detecting knock is the only practical way of seeing how the ignition timing being used influences emissions, power and fuel economy.
1. Cranking and Idle
Some programmable engine management systems have a default cranking advance of 15 degrees, a value about midway through the range of appropriate cranking advances. Smaller engines with faster cranking speeds need a greater ignition advance (up to 20 degrees), while slower cranking speeds of a high compression engine will require less advance (down to 10 degrees). The compression ratio of the engine will also determine the likelihood of kickback on starting. Engines with a low static compression ratio of 8:1 will accept an ignition advance of anything from 0-20 degrees without kick-back. A 10:1 compression ratio will reduce this to 15 degrees, 11:1 to around 10-12 degrees, while race engines using very high compression ratios of 12-13:1 can sometimes tolerate no cranking ignition advance at all.
Most engines will idle happily with an ignition advance of 15 - 32 degrees. This is a very wide range - some engines will certainly not be happy at 32 degrees and others won't be at 15 degrees! An overly high amount of ignition advance for a given engine will result in lumpiness at idle, excessive hydrocarbon emissions and sometimes exhaust popping, while too little advance will also cause lumpiness. If the engine runs closed loop fuel control at idle, too much idle timing advance can disrupt the oxygen sensor reading, causing the self-learning process to overly enrich the idle mixture. Setting the optimal ignition timing can therefore best be done by trial and error variations.
Timing that is more advanced at slightly lower engine speeds than idle is sometimes used to help stabilise idle. This is effective because, when the engine starts to slow down, the greater ignition advance causes the engine to produce more torque, so increasing engine speed. Many factory management systems use ignition timing as a major element in controlling idle smoothness, with an increase or decrease in rpm at idle responded to by a change in timing advance.
Well did u get it?
If the composition of the mixture were constant (and it isn't!), the elapsed time between ignition and full combustion would remain about the same at all rpm. So, if the ignition advance angle were set at a fixed angle before Top Dead Centre, then, as the engine speed increased, combustion would be shifted further and further into the power stroke. This is because the faster moving piston would be further down the bore by the time combustion actually occurred. To prevent this, the ignition advance must increase as engine speed rises.
In addition to engine speed, the other major factor affecting the amount of advance required is the engine load. At light loads (ie when lean mixtures are used) the speed of combustion is slowed and so more ignition advance is needed.
But unfortunately not only does engine speed and load determine the best timing for the combustion of the mixture, but the following factors are also relevant:
the design and size of the combustion chamber
cam timing, especially in variable valve timed engines
the position of the ignition spark(s) in the chamber
the fuel characteristics
the emissions levels required
engine coolant and intake air temperature
the safety margin required before detonation occurs
The emissions of an engine will be affected by the ignition timing that is used, in addition to the air/fuel ratio. Oxides of nitrogen increase as ignition timing is advanced. Running light-load advances of 40 or more degrees is common, giving good responsiveness off load, but if emissions standards need to be met, this advance may have to be reduced. On the other hand, the emission of carbon monoxide (CO) is affected very little by ignition timing, being much more influenced by the air/fuel ratio. At stoichiometric and lean air/fuel ratios, increasing the ignition timing can reduce specific fuel consumption substantially. Finally, the emissions of hydrocarbons at stoichiometric and rich air/fuel ratios increase with advanced timing, but timing has little influence at very lean air/fuel ratios such as 19:1.
It's impossible to ascertain the best ignition timing by juggling all these interrelating factors on paper. Instead, making real-time dyno changes to the ignition timing while using an exhaust gas analyser or air/fuel ratio meter and a means of detecting knock is the only practical way of seeing how the ignition timing being used influences emissions, power and fuel economy.
1. Cranking and Idle
Some programmable engine management systems have a default cranking advance of 15 degrees, a value about midway through the range of appropriate cranking advances. Smaller engines with faster cranking speeds need a greater ignition advance (up to 20 degrees), while slower cranking speeds of a high compression engine will require less advance (down to 10 degrees). The compression ratio of the engine will also determine the likelihood of kickback on starting. Engines with a low static compression ratio of 8:1 will accept an ignition advance of anything from 0-20 degrees without kick-back. A 10:1 compression ratio will reduce this to 15 degrees, 11:1 to around 10-12 degrees, while race engines using very high compression ratios of 12-13:1 can sometimes tolerate no cranking ignition advance at all.
Most engines will idle happily with an ignition advance of 15 - 32 degrees. This is a very wide range - some engines will certainly not be happy at 32 degrees and others won't be at 15 degrees! An overly high amount of ignition advance for a given engine will result in lumpiness at idle, excessive hydrocarbon emissions and sometimes exhaust popping, while too little advance will also cause lumpiness. If the engine runs closed loop fuel control at idle, too much idle timing advance can disrupt the oxygen sensor reading, causing the self-learning process to overly enrich the idle mixture. Setting the optimal ignition timing can therefore best be done by trial and error variations.
Timing that is more advanced at slightly lower engine speeds than idle is sometimes used to help stabilise idle. This is effective because, when the engine starts to slow down, the greater ignition advance causes the engine to produce more torque, so increasing engine speed. Many factory management systems use ignition timing as a major element in controlling idle smoothness, with an increase or decrease in rpm at idle responded to by a change in timing advance.
Well did u get it?
08 April 2004, 11:45 AM
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If it's sent to my home address then not before Monday, as I can't access it from home and am leaving for the weekend at 16:00.
PM me on 22B if you want.
Andrew...
PM me on 22B if you want.
Andrew...
08 April 2004, 12:14 PM
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Excellent reply 
, and much appreciated.
If anyone wants me, I'll be reading it slowly...again
Incidentally, it's my weekend/trackday car, a very modified MR2 Turbo. The turbo now has a T4 compressor wheel and an 8° turbine trim - mmmmmm.
I'm aiming for a genuine 400bhp, which I think is interesting enough in a 2seater midengined RWD hairdressers car
Thanks again
Matt
, and much appreciated.If anyone wants me, I'll be reading it slowly...again
Incidentally, it's my weekend/trackday car, a very modified MR2 Turbo. The turbo now has a T4 compressor wheel and an 8° turbine trim - mmmmmm.
I'm aiming for a genuine 400bhp, which I think is interesting enough in a 2seater midengined RWD hairdressers car
Thanks again
Matt
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