Max power is generally accepted to occur between 12.5:1 and 13:1 AFRs.
People run richer than this to \'cool\' and prevent detonation. However, going from 12.5:1 to 11:1 only cools the inlet charge by 4 degrees for a 13% increase in fuel flow.
If you stayed at 12.5:1 and made up the difference with water, what would the cooling effect be?
Right - from what I have read, not through experience (yet) Bill:
Water has six times the capacity of fuel to cool, or thereabouts.
So, If you want to run 12.5:1 and have the effective equivalent cooling capacity as fuel dumping to 11:1 , you should only need to add around 2 % water / fuel ratio, which equates to around naff all really, somewhere around 50cc/min of water, based on a moderately modded FD.
However, there are other water benefits apparently, not just temperature absorbing, so mixtures tend to be around 10% - 25% water/fuel, which is more like 280cc.min to 700 cc/min. Again based on modded fd typical fuel requirements.
I guess the only way to find out is throw in lots, and wind it back until the car feels right - better done on a rolling road or similar.
Most ideal a/f figures seem to be based on piston engines however.
I\'m not too sure if the unusual shape of the rotary combustion faces and pattern of air and fuel intake would mean that the figures are different.
Again - I haven\'t done this, so I\'m not talking from experience.
Max
Some more:
Every 50cc/min increase in water reduces temps by about 15 celsius looking at an example given by a well known water injection company ;)
Where did the 4 celsius intake temp drop figure come from? I would have expected a higher temperature drop?
Max
Dont just think about water.... Try going for a 50/50 mix with methenol. ;)
Fish
PS. Bill I always thought you had all the answers where this stuff was asked. :p
QuoteOriginally posted by Fish
Dont just think about water.... Try going for a 50/50 mix with methanol. ;)
I think you\'ll find he buys that stuff in 200 litre drums already Danny!
Buy it? Nah, I\'m looking to brew it.
Just really looking to see if anyone is awake.
Anyway the answer is 18 degrees for water. Its all down the the different latent heat of vapourisation of water and petrol.
To give you an idea:
Petrol 350kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Air has a specific heat capacity of around 1kJ/kg
Next question. If the cooling effectof petrol is so minimal, why are turbo cars run so rich?
(this is leading somewhere, I promise).
erm is it cause we\'d all feel guilty if it was cheap to have this much fun? (http://smilies.jeeptalk.org/contrib/geno/rofl.gif)
Si (gets ignorant coat)
QuoteOriginally posted by bill shurvinton
Buy it? Nah, I\'m looking to brew it.
Just really looking to see if anyone is awake.
Anyway the answer is 18 degrees for water. Its all down the the different latent heat of vapourisation of water and petrol.
To give you an idea:
Petrol 350kJ/kg
Water 2256 kJ/kg
Ethanol 904 kJ/kg
Methanol 1109 kJ/kg
Air has a specific heat capacity of around 1kJ/kg
Next question. If the cooling effectof petrol is so minimal, why are turbo cars run so rich?
(this is leading somewhere, I promise).
three degrees out - oh well :)
Because it reduces power? Because it saves on drivers forgetting to add water? Keep coming Bill :)
mmm... I seem to be becoming a dealer in meth ;)
QuoteOriginally posted by 20B_boy
I think you\'ll find he buys that stuff in 200 litre drums already Danny!
Riddle me this then...
As you increase pressure the burn speed increases. Flame propagation is highest near the best power mix, then falls off as you go richer or leaner. So a richer mix will cause ppp to occur later, as will retarding the spark.
So your homework is to look at any knadgered housings you have and see where the detonation that killed them occured.
Reading this thread has made my head hurt! I\'ve still got a whole day of work to do aswell!!
I\'m sure one day this will make perfect sense!;)
Outta intrest what do you guys do for work Engineers/scientists??
I toy at being a manager/director along with trying to answer riddles, not very well I might add!
I\'ll have another look at my housings Friday.....
Engineer. Deranged.
And yes, I have tried to mend a £5 radio. And succeeded
I\'m a complete novice, but this kind of thread is very interesting, wish I knew enough to contribute sensible answers!
Is the ignition timing retarded by the ECU if detonation is detected? Or is it retarded up to a point?
If detonation occurs later with a richer mix, then wouldn\'t the spark plugs have already fired before detonation can occur?
What damage does detonation do to a chamber then? I\'d have though it would be scorching or pitting or something all through the chamber rather than being localised. Perhaps the scorching would be more prevailant on the trailing edge of the rotor with a rich mixture.
QuoteOriginally posted by chrisBoid
I\'m a complete novice, but this kind of thread is very interesting, wish I knew enough to contribute sensible answers!
Is the ignition timing retarded by the ECU if detonation is detected? Or is it retarded up to a point?
If detonation occurs later with a richer mix, then wouldn\'t the spark plugs have already fired before detonation can occur?
What damage does detonation do to a chamber then? I\'d have though it would be scorching or pitting or something all through the chamber rather than being localised. Perhaps the scorching would be more prevailant on the trailing edge of the rotor with a rich mixture.
Coming from a complete novice.....
Some ecu\'s will retard the ignition in given circomstances, Motec I believe and Apexi will retard as the water temp rise above a give deadline, Motec would eventually cut the engine, I think.
Slim chance of detonation if its running rich? If its running richer is the fuel not burning slower?
Detonation can look like someone has hit a rotor surface with a large hammer and dennted it or even in extreme put a hole on it, it certainly can hole a piston. It can either chip or crack apex seals or just shatter them leaving them to exit via the turbo, good bye turbo.
QuoteOriginally posted by g11ary
Coming from a complete novice.....
Some ecu\'s will retard the ignition in given circomstances, Motec I believe and Apexi will retard as the water temp rise above a give deadline, Motec would eventually cut the engine, I think.
Slim chance of detonation if its running rich? If its running richer is the fuel not burning slower?
Detonation can look like someone has hit a rotor surface with a large hammer and dennted it or even in extreme put a hole on it, it certainly can hole a piston. It can either chip or crack apex seals or just shatter them leaving them to exit via the turbo, good bye turbo.
Yes rich mixtures burn more slowly, but you can still get detonation due to a number of other causes. For example, if the spark is too far advanced, then rich or not, PPP will occur early, causing detonation. For my money, less fuel and more spark retard is a less risky strategy. (but happy to be proved wrong). More fuel loses power, and petrol is expensive.
Also, running rich means there is unburned fuel lying around which forms thick carbon deposits. These are prime targets for pre-ignition (as are wrong heat range plugs).
Now think on this. EGR contains lots of water. Think of it as free water injection. When you go single turbo you rip out all the EGR ( this is actually a good thing in the case of the stock setup, but nonetheless interesting to ponder). As I have posted before, by injected COOLED EGR SAAB could run their 4 pots at 20PSI and 14.7:1 AFR.
turbo 13Bs do not like detonation. But the root cause is rarely found. (note send blown engines to Carl for a rebuild. He will take pictures and measurements that should help me find them.
Bill
Sorry..
What is PPP and EGR?
EGR is exhaust gas recirculation. PPP is peak pressure in the chamber.
I think I\'ll have to have a read about the various elements within my engine so I can get to grips with this thread!!
Unfortunatly having masses of work to do isn\'t going to help me achieve this!!