Cranking conditions are different
from when the engine is actually running for several reasons:
The engine speed is much lower, eg 180 – 240
RPM
The engine speed changes throughout the cycle
as each cylinder or rotor does its compression stroke
(that’s the characteristic nyeh sound)
The battery voltage is much lower than when the
engine is running
The ECU determines whether the engine is cranking or
in its running state by comparing the current engine speed to
the cranking RPM threshold. If the engine speed is less than
this RPM threshold then the ECU enters cranking mode which has
different behaviour from when the engine is running, which is
what I will describe in this article. To get an engine to start nicely and reliably, there
are a few considerations. The first, and this is often overlooked but it’s
crucial, is to have a good trigger setup. The earlier the ECU
knows where TDC is and which cylinder it’s up to, the sooner
it can start generating engine synchronous pulses like
ignition and injector events. With the Select ECUs people
would often describe a problem with their setup being “hard to
start” and asking what they should do with the cranking
settings, while all along the problem was they didn’t have a
good trigger setup. If the ECU shows RPM=1 during cranking, this means
that the ECU knows that the engine is turning, for example it
has seen some activity on CAS1, but it doesn’t yet know where
TDC is. As a result it can’t fire any outputs. But we display
the RPM as 1 instead of 0 during this time to help with
problem diagnosis. This means, apart from the fact that it actually has
to work correctly (for example if you have a multitooth and a
reset, then it won’t work if the reset is not working), that
it’s a good idea to have a good trigger for your particular
engine. OEMs generally know what they’re doing, and many OEM
triggers for example the Nissan COP optical and LS1 have
excellent trigger patterns which can tell the ECU where it is
within the 720 degree cycle with 180 degrees of crank rotation
or less. If you’re going to make changes to the OEM system
then you need to understand the consequences of those changes
– for example if you wire up a non-VVT 2JZ engine and only
connect one of the cam triggers then it could take up to 6
nyehs before the ECU even knows where TDC is. Or if you
convert a distributor type Nissan optical system to coil on
plug, the engine may need to crank for up to a full cam
revolution for the ECU to know which cylinder to fire (which
is not an issue on the distributor system because the
distributor handles that). Next, we will consider battery voltage. The main
effect of this is that there’s less voltage available to make
the injector open. The injector needs a certain amount of
force to open against the fuel pressure, and that force comes
from the injector current. The current comes from the voltage,
so at a lower battery voltage there’s less force available.
This means that the injector dead time has to be set correctly
against different voltages, which is why we recommend using
injectors which we have characterised. One trap I’ve seen some
young (and sometimes not so young) players fall into is to
take a car which had low impedance injectors from the factory,
with a series resistor pack in the original wiring – and
change the injectors to high impedance type but leave the
resistor pack in. If you remember Ohm’s law it will be obvious
why this will cause problems, but the problem is that
sometimes there’s still enough current with the resistor pack
installed for the injector to open at 14V, but not at 10V or
less when the engine is cranking. This can lead the
inexperienced tuner to believe that there’s a cranking fuel
problem. When the engine is stopped, and during cranking, the
ECU will hold the idle valve open at 100%, to make the engine
easier to start. This means that on a PWM idle control valve,
it will be fully open. On a stepper motor idle controller, it
means the ECU will fully retract the plunger which also has
the benefit of homing the stepper motor. On an electronic
throttle car, it means that the target throttle position will
be the idle authority plus whatever the pedal is commanding –
eg if the idle authority is 20% and the pedal is at 0%, then
the target throttle position will be 20%. Finally, we will discuss ignition and fuel delivery
during cranking. Ignition timing is controlled by a map of target
ignition timing vs coolant temperature. The ECU may not have a
good indication of the engine angle as the engine rotates,
because as I mentioned before the engine speed changes a lot
throughout the cycle. So the ECU will try to time the pulse
based on that angle, but if it gets to a tooth past that angle
and hasn’t fired yet, it will generate a pulse. So this means
that pulses can be generated based on the availability of a
trigger event close to TDC. Most engines are happy to fire at about TDC during
cranking – sometimes earlier, on some engines up to 10°BTDC.
Some engines really don’t like this though, especially high
compression ones, and sometimes firing too early can “stall”
the starter motor. On some engines I’ve even heard of starter
motors breaking and being flung off engines when this has
happened. So before firing up an engine it’s usually a good
idea to disable the fuel to the engine and crank just with the
ignition, and check the timing with a light. Lastly, we’ve come to the fuel delivery during
cranking. This comes in two stages; the first is a prime pulse
that fires on all the injectors at once to get an initial
amount of fuel into the engine and wet the runner walls, and
after that the injection pulses are delivered synchronously
just as when the engine is running. The first table we will look at is the Cranking Map,
which you can find under fuel tuning -> cranking. This is a
simple VE table (or you can override it to b e in milliseconds
if you wish) against coolant temperature. If you are running
flex fuel, then you will have one of these for E0 and one for
E85. This is the VE of the engine at cranking, to get the
right amount of fuel into the engine tog et it to fire.
Normally we see values of about 80% when the engine is warmed
up, and higher when the engine is cold, for example 150 – 200%
on petrol / gasoline. On ethanol the cranking fuel needs to be
even higher, for example 300%. If you need to adjust the number and you’re not sure
which way to go, here’s the technique I use. If the engine has
too little fuel, then putting a bigger number in there will
make it fire up more quickly. If it’s either very rich on
immediate post-start, or it fails to start until you open the
throttle and then it fires up, then that means you have too
much fuel. The next table we’ll talk about is the prime gain.
This is a 3D table against coolant temperature and ethanol
percentage. This is an asynchronous pulses on all the primary
injectors at once, when the ECU first detects RPM. Note that
it doesn’t need to know where TDC is, it just needs to see
some activity on the CAS1 input. This prime gain is a
percentage of the cranking pulse. So for example if your
normal cranking pulse width is 10ms, and you put 50% in the
prime gain table, then the ECU will do an asynchronous pulse
of 5ms (assuming dead time = 0) when it detects the engine has
started to rotate, on all the primary injectors. The final tables we’ll discuss are what happens
after the engine fires up. The variable called “Time since
starting engine” will start to count up from zero (in
seconds), and this is used to reference two tables. One of
them is the post crank enrichment – whose two axes are time
(since starting the engine) and coolant temperature. The
values in the table are then the percentage enrichment
required. The other table has the same axes and that’s the
post-crank idle-up function. This additional idle effort gets
added to the other idle calculations. While the ECU is in
post-crank idle-up, which means that the interpolated value
from the 3D map is greater than 1%, the ECU will not go into
closed loop idle or overrun fuel cut.
Functions>Idle Stepper>Base Idle>Post Crank Idle
There are a few variables to get right, but once you
do it’s nice to have an engine that starts like an OEM engine. Thank you.