Injector size estimation (low speed engine)

[seagah]

Hi people!

I have a six-cylinder naturaly aspirated inline engine with following
parameters:

Bore: 4.13 in (105 mm)
Stroke: 4.53 in (115 mm)
Swept Volume: 364.31 cu in (5.97 L)
Compression Ratio: 6:1
Power at 2300 rpm: 143 bhp (106.6 kW)
Power at 2600 rpm (max allowed): 152.9 bhp (114.0 kW)
Consumption: 13.5 gal/hr (51.2 L/hr)

I use it for a long time on my powerboat (water-jet). I have drown it last autumn...
Not a place for a sad stories, so, I have the Megasquirt-3 with MS3X fully assembled and tested.

Now I am trying to get right calculation results to choose the injector's size. So, take a look a this:

1. Using the online calculator to estimate the injector size needed
(http://www.megamanual.com/v22manual/minj.htm) I got the flowrate = 15 lbs/hr.

2. Then I have tried to compute the Req_Fuel value
(http://www.megamanual.com/v22manual/mfuel.htm#inject) and got 42.5 ms!!!

3. The intake stroke for the engine takes 270 degreeses of a cycle (720
degreeses) wich is 37.5% of the cycle time. It's only 62.5% of the cycle time available to squirt.

4. At 2600 rpm the stroke cycle gets 60sec / 2600rpm * 2round/cycle = 0.046sec (46ms).

5. Well, in addition, I tried to estimate the VE of the engine
(http://users.erols.com/srweiss/calcve.htm) and got VE = 144.9%

Which injector size is suitable for my project? How do I compute the right flowrate for a single injector? Should I increase the flowrate needed by the coefficient to consider the computed VE (1.5 my guess)? Which the Req_Fuel time is optimal for my engine?

Much thanks and best regards to all you.

seagah

[billr]

Stop using the Megamanual, the info there is not up-to-date, maybe even completely wrong, for MS3.

However, those injector size and req_fuel values seem about right. You are using six injectors and running gasoline, correct? Since the req_fuel is so long (OK for this low speed engine) you could use larger injectors with no problem, you will be a long way away from the trouble-some 1-2 msec pulsewidths!

As to the VE table, whatever you "calculate" is only going to be a starting point, particularly with a non-common engine design like this. You could probably set every VE cell to "75" and be as close as anything you can calculate.

[seagah]

Thank you billr!

I use megamanual for educational purposes only (as theory source). As for hardware, configuration, tuning etc. I use http://www.msextra.com/manuals/ms3manuals/. Msextra is poor of theory in my opinion.

I am going to use Bosch 0-280-150-560 (20.0lbs/hr at 43.5 psi). Is it suitable for my project (just to be fully ensure)?

Much more thanks for your advise on VE table configuration.

Best regards
seagah

[billr]

Those 20# injectors should be fine, maybe even a better choice. I have no good handle on the BSFC for a 6:1 engine, but expect it is a bit more than "usual". You are going to use six injectors, not two on a couple of TBI units in place of the carbs I think I see in the photo?

[seagah]

Yes, you are right. It's carbs you can see on the photo. I'm going to use the sequential injection (six injectors and one squirt). It's why I assebled ms3 with ms3x. With carbs the engine guzzles down too much of fuel. A point for me is to repair the engine, to reduce consumption of fuel and, as a result, to increase the action radius for my powerboat. Thank you, billr. It's now all clear for me.

Best regards!
seagah

[billr]

To improve fuel economy, one of the first things to consider is raising the compression ratio, and maybe the prop pitch to make sure the engine is running with a high MAP in most operations. Consider that the theoretical thermal efficiency of an Otto-cycle (spark ignition) engine is about 50% at 6:1, that rises to 60% at 10:1. Throttling the engine effectively reduces the CR it is operating at, hence the desire to keep MAP high. Also note that this trivia applies mainly to NA engines, running boost changes the picture, but never to get increased thermal efficiency.

[seagah]

Yes, you are right again! But increasing of the engine's compression ratio cause a change of the hp/rpm characteristics. Please, look at the picture attached. The curve 1 is hp/rpm characteristics of non forced engine, the curve 2 - the same but for a forced one. The Nwj curve - is a hp/rpm characteristics for a water-jet pump (power demand). As you can see, forced enjine gives less power and rpm on the pump. It's a water, not gears.

I expect a reduction in fuel consumption due to the correct table setting for cruising mode.

Thank you, billr.

--
Best regards,
seagah