Meaning of Run RPM

[mahicshah]

We are a Formula Student team, and we are using the MoTeC M400 ECU with our Honda CBR600RR PC37 engine (2005 model).

What exactly does the 'Run RPM' mean? It is located under Adjust--General Setup--Miscellaneous Setup. Does it mean that the engine won't crank unless the starter motor rotates the crankshaft at a speed higher than the 'Run RPM' that I have entered? I have currently set it at 200 RPM.

Also, we are using a 12V 14Ah lead-acid battery with my system, and the starter motor drains it extremely fast. Even with the stock alternator and rectifier connected, the engine can't be cranked more than thrice on a fully charged battery. All the electronic components on our car require about 20A total at any given time, and this also causes the battery to drain very fast. Are there any parameters in the ECU to help crank the engine faster? Will changing the Run RPM to a lower value crank the engine faster?

[Stephen Dean]

Hi,

Run RPM is the threshold where the engine is self supporting, i.e. it isn't being turned by the starter motor and is running under it's own combustion cycle. This value is used to determine when the engine is using the cranking fuel/ignition settings or is in post start or run states. This is typically set midway between the typical speed that the starter cranks the motor at and the set idle speed.

Do you need to have as much load on the battery at the time of cranking? It isn't that high a capacity battery that you are using either. The best way to get the engine to start faster (on the first or second rotation) is to ensure that the tuning is correct in that region, that the Ref Sync settings are correct so that the ECU Sync's up as quickly as possible and that the cranking and post start compensations are correct.

[Sean]

All the electronic components on our car require about 20A total at any given time, and this also causes the battery to drain very fast.

If I may ask, what electronics do you have that's giving you such a large draw?

[mahicshah]

Hi,

Run RPM is the threshold where the engine is self supporting, i.e. it isn't being turned by the starter motor and is running under it's own combustion cycle. This value is used to determine when the engine is using the cranking fuel/ignition settings or is in post start or run states. This is typically set midway between the typical speed that the starter cranks the motor at and the set idle speed.

Do you need to have as much load on the battery at the time of cranking? It isn't that high a capacity battery that you are using either. The best way to get the engine to start faster (on the first or second rotation) is to ensure that the tuning is correct in that region, that the Ref Sync settings are correct so that the ECU Sync's up as quickly as possible and that the cranking and post start compensations are correct.

Thank you for the quick reply. I do switch off the fan and the water pump for the instant when I crank the engine, and then switch them back up immediately after. It does help, but I just get around 1-2 more cranks by doing so. I have noticed that the starter motor drains the battery tremendously fast, and wanted to find a way to fix that. I can't use a higher capacity battery due to space constraints.

What changes should I make to the current tuning to make the engine start faster? The Ref Sync settings are correct, but I don't have an idea of the post start compensations. Could you please help in that as well? Thank you!

[mahicshah]

All the electronic components on our car require about 20A total at any given time, and this also causes the battery to drain very fast.

If I may ask, what electronics do you have that's giving you such a large draw?

These are the major components and the amount of current that they are drawing continuously (checked with a clamp meter):
Bosch 4.2 LSU - 2A
Perma-Cool 19120 2350CFM Fan - 5A
Water Pump (not sure of which brand) - 3A
GSL393 Fuel Pump - 5A

Apart from these, we have a custom dash (including shifter lights), relays, DC-DC converters, and also a custom data acquisition system.

When I put the clamp meter around the negative terminal of the battery, it displayed around 20A.

[Stephen Dean]

Only the Fuel Pump needs to be running when cranking, the others can be powered up after the engine is running.

[mahicshah]

Only the Fuel Pump needs to be running when cranking, the others can be powered up after the engine is running.

Yes, I switch everything else off while cranking, that is what I mentioned earlier as well. How can the post start compensations help in this, though?

[Stephen Dean]

The better the Cranking and Post start compensation tuning is, the quicker that the engine will stabilise at its Idle aim point. If the engine is struggling to start as it doesn't have enough fuel to sustain complete combustion, or hunts whilst the fuel film is building up in the inlet runner before it will go into idle, then the alternator is not producing voltage to maintain the battery charge so all of the devices that are running off of the battery and drawing power out of the battery, without the load being compensated for by the alternator.