## Vapor LPG injection using Speed-Density control algorithm

## Vapor LPG injection using Speed-Density control algorithm

(OP)

Hello All. There is a vapor LPG port fuel injection. Speed-Density air mass estimation is used. However, my calculations seems to be incorrect (the calculated injector pulse width does not produce a correct mixture) and I do not know what can be wrong. I really appreciate any kind of review or double check. For example, when engine is idling:

Cylinder volume incl. combustion chamber (V): 0.388 l

Manifold air pressure (P): 40 kPa

Volumetric efficiency (VE): 0.53

Manifold air temp (Ta): 23°C (296.15 °K)

Molar mass of air (Ma): 28.97 g/mol

Gas constant: 8.314 J/°K/mol

Estimated airmass used per one working cylce (ma):

ma = (VE * P * V * Ma) / (R * Ta) = (0.53 * 40E03 * 3.88E-04 * 28.97) / (8.314 * 296.15) = 96.8 mg of air per cycle

Fuel mass (mf): (Stoichiometric AFR for LPG is 15.7)

mf = 96.8 / 15.7 = 6.16 mg

Injector specifications by the manufacturer:

Static flow (isf): 38 SLPM (Standard liter per minute, means at 0°C and 101.325 kPa), @ 1 bar differential prerssure

Offset (io): 0.79 ms @ 14 V, 1 bar diff. pressure

Conditions:

Supply voltage = 14 V

Fuel pressure = 1.4 bar (this gives a diff pressure 1.0 bar)

Fuel temperature (Tf): 50°C (323.15 °K)

Molar mass of vapor LPG (Mf) (assuming 1:1 propane:butane) = 51.11 g/mol

Calculate the required fuel (LPG vapor) volume (Vf) at 50°C and 101.325 kPa:

Vf = (mf * R * Tf) / (Mf * P) = (6.16E-03 * 8.314 * 323.15) / (51.11 * 101325) = 3.196 ml = 3.196E-03 liters

Then, calculate the required injector pulse width (IPW):

Static flow (isf): 38 SLPM = 0.6333 SLPs

IPW = Vf / isf / 60 + io = 3.196E-03 / 0.6333 + 0.79E03 = 5.836E-3 s = 5.836 ms

HOWEVER, on the real engine, I have to open the injectors for approx. 10 ms to achieve lambda = 1. I also tried to measure the dynamic behavior of the injectors and the results are almost same as the manufacturer reported. So it seems I calculated something wrong what I do not see.. Do you see that?

THANKS A LOT

Cylinder volume incl. combustion chamber (V): 0.388 l

Manifold air pressure (P): 40 kPa

Volumetric efficiency (VE): 0.53

Manifold air temp (Ta): 23°C (296.15 °K)

Molar mass of air (Ma): 28.97 g/mol

Gas constant: 8.314 J/°K/mol

Estimated airmass used per one working cylce (ma):

ma = (VE * P * V * Ma) / (R * Ta) = (0.53 * 40E03 * 3.88E-04 * 28.97) / (8.314 * 296.15) = 96.8 mg of air per cycle

Fuel mass (mf): (Stoichiometric AFR for LPG is 15.7)

mf = 96.8 / 15.7 = 6.16 mg

Injector specifications by the manufacturer:

Static flow (isf): 38 SLPM (Standard liter per minute, means at 0°C and 101.325 kPa), @ 1 bar differential prerssure

Offset (io): 0.79 ms @ 14 V, 1 bar diff. pressure

Conditions:

Supply voltage = 14 V

Fuel pressure = 1.4 bar (this gives a diff pressure 1.0 bar)

Fuel temperature (Tf): 50°C (323.15 °K)

Molar mass of vapor LPG (Mf) (assuming 1:1 propane:butane) = 51.11 g/mol

Calculate the required fuel (LPG vapor) volume (Vf) at 50°C and 101.325 kPa:

Vf = (mf * R * Tf) / (Mf * P) = (6.16E-03 * 8.314 * 323.15) / (51.11 * 101325) = 3.196 ml = 3.196E-03 liters

Then, calculate the required injector pulse width (IPW):

Static flow (isf): 38 SLPM = 0.6333 SLPs

IPW = Vf / isf / 60 + io = 3.196E-03 / 0.6333 + 0.79E03 = 5.836E-3 s = 5.836 ms

HOWEVER, on the real engine, I have to open the injectors for approx. 10 ms to achieve lambda = 1. I also tried to measure the dynamic behavior of the injectors and the results are almost same as the manufacturer reported. So it seems I calculated something wrong what I do not see.. Do you see that?

THANKS A LOT

## RE: Vapor LPG injection using Speed-Density control algorithm

I find your assumed manifold air temp questionable - at idle, it can be quite a bit warmer than ambient, due to heat transfer.

For the same reason, your assumed fuel temp may be low.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Vapor LPG injection using Speed-Density control algorithm

## RE: Vapor LPG injection using Speed-Density control algorithm

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Vapor LPG injection using Speed-Density control algorithm

je suis charlie

## RE: Vapor LPG injection using Speed-Density control algorithm

## RE: Vapor LPG injection using Speed-Density control algorithm

je suis charlie

## RE: Vapor LPG injection using Speed-Density control algorithm

VE across the intake valve, in the definition familiar to me, is a strong function of RPM and a weak function of conditions in the cylinder, intake manifold and exhaust manifold.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Vapor LPG injection using Speed-Density control algorithm

Absolute air pressure in the rail: 196 kPa

Barometric pressure: 96 kPa

Ambient temp: 12° C

Supply voltage of ECU: 14.0 V

Where:

IPW = Injector pulse width

CNT = Count of the pulses

ml/imp = measured air volume per one pulse at 12° C and 96 kPa

sml = measured volume per one pulse recalculated to 0° C and 101.325 kPa // sml = (ml * (273.15/(273.15+12))*(96/101.325))

Results:

Static flow is 0.5528 SLPS (standard liter per second)

Offset is -0.5504 ms .. This is a bit strange to see a negative offset. I think it means that the open time is shorter than the close time. There is a peak&hold type injector.

However, to deliver above mentioned 3.196 sml of the gas, I need to open the injector for (3.196 - 0.5504) / 0.5528) = 4.786 ms .. Looks plausible compared to above calculated 5.836 ms, since the injector manufacturer does not specify a dynamic behavior. They specified just complete open time, complete close time and static flow. This means the above calculation does not consider that the gas flows also during opening and closing phase of the injector.

But, still don't know why my engine needs 10 ms open time under this condition. :D

Questions:

Can I make a mistake using an air instead of vapor LPG? I don't think so since I measured a volume.

Could the things change when the injection is performed not from approx. 2 bar (rail) to 1 bar (manifold), but from e.g. 1.4 bar to 0.4 bar? I don't think so since the diff pressure is the same.

## RE: Vapor LPG injection using Speed-Density control algorithm

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

## RE: Vapor LPG injection using Speed-Density control algorithm

## RE: Vapor LPG injection using Speed-Density control algorithm

@mados650 How was the VE determined?

je suis charlie

## RE: Vapor LPG injection using Speed-Density control algorithm

You have mentioned the alternative Speed-Density algorithm which is also commonly used. Can you describe this more in detail?

http://www.useasydocs.com/theory/vetable.htm

## RE: Vapor LPG injection using Speed-Density control algorithm

BTW, I agree critical flow conditions at the injector nozzle is the probable cause of your miscalculation.

je suis charlie

## RE: Vapor LPG injection using Speed-Density control algorithm