Saabscene Saab Forum - Saab Technical Information Resource banner

1 - 4 of 4 Posts

·
Registered
Joined
·
25 Posts
Discussion Starter #1
having read a bit about water injection i found this site whilst looking for somthing compleatley unrelated. They seem to be having a rarther livley debate about the "possible" effects, I have to say that the maths went zooming well above my head but they have some vey interesting ideas & data here is just one post that poses a point i never thorght of

Quote"It requires less work to compress cool air than hot air.

I did the calculations comparing the amount of work it takes to do a 10:1 compression on air at 70 deg F and 110 deg F.

At 70 deg F: 88.7 BTU
At 110 deg F: 94.9 BTU

For an engine running 20 pounds of air a minute (roughly the amount of air a 5.9 liter "Ram Diesel" with a turbocharged engine runs at 1800 rpm), the horsepower the engine uses compressing air during the compression stroke is as follows:

At 70 deg F: 88.7 hp
At 110 deg F: 94.9 hp

Using some engine simulation software I compared the engine brake cylinder efficiency at the two temperatures. At 70 deg F the efficiency was 43.27%; at 110 deg F it was 42.65%. For a diesel engine flowing at 20 lbs/min this would work out to about 2.5 hp.

It appears to me that another effect of water injection may be to decrease the charge temperature and therefore decrease the amount of work the engine has to do during the compression stroke."

http://www.eng-tips.com/gviewthread.cfm/le...id/71/qid/72284

and heres a few questions from me

dose any one have any thorghts on pre & post turbo injection?

would a fuel pump & injector be a good starting point ?

or is manifold presurisation of the water resivoir more favirouble?

hope you enjoy the thread i found
enjoy & happy

loj
 

·
Registered
Joined
·
1,448 Posts
First off, pre-turbo water injection is great for race cars which are not allowed intercoolers and which don't have to last terribly long. But injecting water before the intecooler can in some instances be somewhat inneficient, and the water droplets can "pit" the turbo compressor wheel after long periods of time.

Think of this:

1. Inject before the intercooler, cools it down ambient temp before it reaches the intercooler (this is exagerrating as it doesn't cool that much, but you get the point). Then as it passes through the intercooler what happens? Nothing it's already at ambient temp and the intercooler can't transfer any heat.

2. Inject after the intercooler, intercooler gets it down to ambient temp (again exagerrating to make more clear) then the water injected cooles the ambient temp air down well below ambient temp, giving you the coolest charge temps possible.

As for fuel pumps and fuel injectors, to my knowledge they do not sufficiently atomize water, and are not designed to be used with water. They could be damaged by it. Aquamist makes a very good injection system which, while not terribly cheap, is quite effective.

Now for some water injection theory ...

Linkoping university in Sweden has done some work with water injection experiments. They noted that a properly tuned ECU could easily get 10% more power from most engines just by adding water injection. What's even more interesting though, is that water injection effectively raises the octane number of your fuel. Higher octane fuels are harder to ignite, and burn slower, thus adding water injection to lower octane fuels gets the same effect as the higher grade stuff. It's much less likely to pre-ignite or detonate.

It should be noted however, that the good fellows at Linkoping found that putting water injection on a car without a specially designed ECU will actually cause a slight decrease in power and fuel mileage. Since humidity has a large effect on air density, and most engines have no system to measure or compensate for it, the drastic increase would throw off the computer. That, and because they tune engines to run on a specific octane level for peak efficiency, the increase in effective octane changed the location for ideal spark advance. (When they did the experiment the denser cooler air fooled the computer into thinking it was under more load, and so it retarded the ignition timing and richened the mixture, when in fact it should have advanced the timing and leaned the mixture slightly. Once they compensated on their own manually fuel consumption was down about 10% and power up 5%.)

Before you get overly pessimistic though, putting water injection on a high performance turbo car, irrespective of custom tuned ECU, will "allow" for more power even if the addition itself doesn't make any. If you have your water injection on at low boost you'll lose power, but it will raise your knock limit, allowing you to run more boost than without it. The ideal solution of course is to have a custom tuned ECU which would not only allow more boost at WOT, but would lean the mixture and run more advance at part throttle, giving you better mileage and power without booting the "go pedal" too hard . But as far as I know no one makes such an ECU. Could be coming in the near future though so keep on looking. In the meantime, there are ECU's that are tuned for high octane. It might work out that by installing water injection you can run your car at the same power level you have now, but on lower octane fuel, which would save you money at the pump since water is a lot cheaper.

Cheers,
Dubbya
 

·
Registered
Joined
·
8,948 Posts
and the water droplets can "pit" the turbo compressor wheel  [/b]
Would this be caused by the localised low pressure boiling of the water droplets passing the aerofoil blades of the turbo?

i.e what in badly designed boats is called cavitation?

Andrew
 

·
Registered
Joined
·
1,448 Posts
It's more likely just that the droplets hit the aluminum so hard they run out its fatigue life and thus dent it. But I also think it could be that they knock off the protective oxide layer and doing so continuously slowly errodes the metal. Remember aluminum does oxidize, very strongly in fact, but is protected normally by an oxide layer that develops quickly in the presence of air and can be knocked/scratched off.

As for cavitation ... in boats that's due to the inertia of the water around the prop, and the fact that water has a reasonable vapor pressure. However the decrease in pressure necessary to get this effect would require an extremely inefficient turbocharger. The turbulence of the air may effectively rip the droplets apart when in the turbo ... but the pressure would have to drop to nearly zero for the water to just boil away.
 
1 - 4 of 4 Posts
Top