Simulink - Vehicle Acceleration 4
2. Model Development
This model is only sightly different to the previous one. The one dimensional look up table block is replaced by the two dimensional look up table block. One input to this table is still the engine speed obtained from the feedback loop taken from the vehicle speed. The other input is from the signal builder block used to synthesise the specified throttle positions - going from shut to fully open in 1 second then remaining fully open for the remaining 9 seconds of the simulation.
The major task is to construct the contents of the look up table. Curves showing
how engine torque varies with engine speed at full throttle are published for many
engines, however part throttle curves are not normally available. Some estimation
(or guesswork) is required to derive appropriate data. For throttle openings of
1/4, 1/2, 3/4 it is probably reasonable to assume (for engines that are not turbo-charged)
that the engine torque is pro - rata that of the full throttle torque at the
same engine speed. Deciding on a value at small throttle openings is less
straight forward. Putting in zero torque means the simulation never gets started.
putting in a small nominal value, say 1, gives a low initial acceleration. It is
probably best to put a small nominal value for closed throttle and then a larger
value for 0.1 fully open throttle.
One point that should be made is that in practice a car with a manual transmission will have the engine revved somewhat while the clutch is released to pull away.
The closed throttle engine torque values chosen were 10 Nm for all engine speeds. The data put in the table is given below:
Again the model is limited by only having 1 gear ratio and the engine rpm at 6 seconds is about 8000, which is too high for the engine the concerned. The time to reach 60 mph is indicated as just under 6 seconds which would not be achieved in practice!
David J Grieve, 15th October 2004.