Injecting prior to turbo comp' impellers

[leman_opc]

Just a couple of thoughts why I may not be seing any gains in air consumption:
1. My flow issues and hence simply not enough water injected pre-turbo.
2. I was looking at max air consumption but not at the whole curve. It may have been that the air consuption gains are there but not at the peak point.
3. The whole concept is around the compressor efficiency gain, but it has been correctly noted that the energy consumed by the water evaporation in the compressor is still produced by the turbine part, hence the efficiency gain of the turbo in whole is debatable. Based on above logics it may occur that if the compressor part is a restriction for more flow then preturbo injection may have better results compared to the case where the exhaust mainofld is restrictive, in the latter case the effect may be only minor (if any).

The first 2 points may be remedied. I'll double check.

The last item is hard to verify but I believe the high exhaust manifold backpressure could be my case.

[Liborek]

Originally Posted by leman_opc

The whole concept is around the compressor efficiency gain, but it has been correctly noted that the energy consumed by the water evaporation in the compressor is still produced by the turbine part, hence the efficiency gain of the turbo in whole is debatable.

This assumption would apply in post turbo setup - if compressor works at certain mass flow and PR with correspoding efficiency, introducing injectant after it can only decrease manifold pressure for given flow rate, or increase mass flow for given pressure.

Pre-turbo injection shifts actual compression process from adiabatic closer to isothermic, and thermodynamics says that later process requires less energy.

To actually increase peak flow, you would have to increase up-stream pressure, or decrease up-stream temperature, and considering usual ambient temperatures, one can't achieve significant temperature drop in pre-turbo location without methanol.

Otherwise you can read-up papers on water injected turbines, IIRC they don't have too much increase in peak flow conditions, but turbine work requirement for given mass flow is decreased by very substantial amount which results in net increase in power and efficiency.

Lotus did development work of pre-compressor fuel injection (gasoline and E85) on supercharged 1.8 4 cylinder and they were able to increase BMEP due to charge cooling effect and also net power, torque and efficiency level were increased as supercharger consumed less crank power.

Thing is, that reduced compressor work on turbocharger, can be observed as reduced backpressure, but if the engine isn't much sensitive to it, or compressor doesn't work in some boundary condition to begin with, or both, not much gains in actual power can be observed.

There is actually test of pre-turbo vs. post-IC injection over rx7club, pre-turbo setup made about 25 rwhp more at peak power (500HP level), without significant change in AFR or injection pulse width - essentially same air mass flow. So whole power increase has to come from increase in overall efficiency through reduced pumping losses against turbine.

[leman_opc]

Originally Posted by Liborek

Pre-turbo injection shifts actual compression process from adiabatic closer to isothermic, and thermodynamics says that later process requires less energy.

The isothermic compression process requires less energy, but the energy spent on water evaporation is on top of that, correct? At least that's what I got from the last pages of this thread...

Anyway, I believe you are saying the actual case is opposite to what was my guess (i.e. there should be no major peak flow increase, but less energy spent on the compression process -> less backpressure, and hence gains if the backpressure was an issue, and maybe no gains if the backpressure was low to begin with)?

[Liborek]

Originally Posted by leman_opc

The isothermic compression process requires less energy, but the energy spent on water evaporation is on top of that, correct? At least that's what I got from the last pages of this thread...

I didn't thoroughly read last pages but no, you got it other way around.
Energy required for evaporation of water, or any other liquid for that matter, is drawn in form of heat from the actual process. That heat is free. Water simply absorbs heat of compression and according to ideal gas law, keeping volume constant (lets consider one running condition) while decreasing temperature will decrease pressure, so compressor will internally work with lower pressure ratio and hence, lower power requirement. One has to account for increased mass of water introduced into compressor and certain change in specific heat ratio for mixture of air and injectant vs. air alone. Some of these factors increase turbine power requirement, some decrease, but in every case I have seen, sum of these factors always decreases turbine power requirement.

If it wasn't the case, all the proved examples from the real world wouldn't work in the way they do

Originally Posted by leman_opc

Anyway, I believe you are saying the actual case is opposite to what was my guess (i.e. there should be no major peak flow increase, but less energy spent on the compression process -> less backpressure, and hence gains if the backpressure was an issue, and maybe no gains if the backpressure was low to begin with)?

It depends if the backpressure is high due to restrictive hotside, or if it "has to be" high to supply compressor power requirement. In later case, we can imagine overspeeding compressor at peak flow working with poor efficiency. Introduction of water would decrease work requirement so backpressure would be lower.

If compressor works with good efficiency but turbine is restrictive, introduction of water won't make much difference - even if it would decrease temperature of exhaust gas, this decrease in thermal energy must be made up by mass flow or pressure, and since we would be limited by air+fuel+water mass, pressure would again increase to reach equilibrium.

Problem of proving the concept in automotive turbocharged engine lies in fact, that even if you would decrease backpressure by large amount, say 20% in absolute value, it would increase engine power by 5% and only in case of engine which is very sensitive to backpressure, like wankel rotary, and if it was high in the first place. And such increase in power can easily be in the range of error of measuring device etc. And of course, water flow rate to reach such theoretical values is quite substantial...

Nice thing about RX-7 example is the fact, that pre-turbo injected car actually runs faster on track and then, all the theory comes to an end :-)

[leman_opc]

Thanks for the detailed explanation on this.

Based on what you say I assume with preturbo injection the result may be close to the following options:
1. Boost decrease but virtually no gains in air consumption (in case of restrictive hotside being an issue), or
2. Unchanged boost but increased air consumption (in case the hotside may still flow the increased air mass)?

[Liborek]

Originally Posted by leman_opc

Thanks for the detailed explanation on this.

Based on what you say I assume with preturbo injection the result may be close to the following options:
1. Boost decrease but virtually no gains in air consumption (in case of restrictive hotside being an issue), or
2. Unchanged boost but increased air consumption (in case the hotside may still flow the increased air mass)?

I assume that your setup is intercooled, correct?

Intercooler is very efficient device and it largely diminishes difference in temperature of charge air which would be caused by different compressor efficiency, so when you measure some absolute pressure and temperature in intake manifold, you know it has certain density and it doesn't matter how this state has been achieved.

So don't dwell that much on airflow number. Setup without intercooler would produce more obvious difference in power in regards to position of water injection.

Engine net brake power isn't just about combustion of air and fuel, but also about minimizing losses. Very high back-pressure not only impedes airflow, but its also direct negative work for engine. Unfortunately, I really don't know how much power could be "freed" in reducing one, some complex engine simulation could do it, we must live up with just performance testing :-)

[leman_opc]

Yes, this is an intercooled setup. True - I've been more than happy with my intake temps when injecting only post-IC, so hardly any gains here.

It's still puzzling for me that no gains in aiflow are seen due to expected backpressure drop. I follow your logics above re. minimized engine losses, so probably there are gains in power output even despite unchanged air consumption, but I still would expect at least some (probably faint, but still) air consumption increase if there is any change in engine flow characteristics when injecting pre-turbo.

Anyway, I have now my car back from the workshop, so more testing in the next days. The photos of the compressor wheel have been made also to see if there will be any wear. I will also have my car custom mapped by the end of the month so will see if any gains are seen on the dyno.

[leman_opc]

Some more testing today after I sorted my flow issues (it appears that a dual nozzle setup with a checkvalve on one nozzle only is a no-no, now I put in a checkvalve for each nozzle). Had two different runs:
1. 0,5 mm nozzle preturbo, 0,3 mm nozzle post-IC. 805 kg/h air consumption, knock retards up to 9 degrees.
2. 0,5 mm nozzle preturbo, 0,5 mm nozzle post-IC. 805 kg/h air consumption, knock retards peaking 8 degrees in 1 cylinder, but generally up to 5 degrees.

So, the major outcome is that the concept finally works, 2% increase in air consumption is not much but the pure fact is important. Furthermore it has been noted above that the gains may be higher power-wise.

Previously I obviously was not injecting enough preturbo due to flow distribution problems. Will have a more detailed look on the logs next days.

[Liborek]

Originally Posted by leman_opc

So, the major outcome is that the concept finally works, 2% increase in air consumption is not much but the pure fact is important. Furthermore it has been noted above that the gains may be higher power-wise.

Previously I obviously was not injecting enough preturbo due to flow distribution problems. Will have a more detailed look on the logs next days.

I assume that airflow numbers are STP corrected?

In your case, airflow increase is not high since you already pointed out boost drop in high rpms which indicate that compressor is out of flow. Any increase is possible only via decreased temperature pre-turbo. Bigger difference would be seen with water-methanol mixture.

[leman_opc]

Since its mass air flow I'm not sure STP correction is relevant here... Anyway it's just the data I log from the MAF sensor and I'm not really aware which adjustments are there.

On the last point - I was actually expecting to see the flow capacity of the compressor expanded, this was the core idea behind the test. I'm now thinking whether a bigger jet pre-turbo is worth trying. It makes sense but on the other hand should be probably done with more advanced atomisation than a regular jet...

I'm currently injecting 40% ethanol 60% water and would really like to keep this mixture while the system is in test mode so to say (just don't want to deal with meth while I'm changing jets etc...).