Fuel and spark

[DVS2XS]

 

Engine Management Primer

 

 

The following is a basic overview of how electronic engine management systems work. It is not meant to be all inclusive, but rather to give a basic understanding of what goes on in the electronic control unit (ECU) or Processor Control Module (PCM).

 

The basic systems are centered on a chip or processor. Many U.S. automakers use processors from Motorola like the 555 (triple nickel), 563 (silver oak), 565 (spanish oak) etc. These are 32 bit processors that can read and adjust many different sensors and actuators several times each second. Today many sensors are digital, meaning that they range from 0 to 5 volts and the voltage sent to the ECU is mapped and lets the ECU know exactly how much voltage corresponds to each given input for each sensor. A change in 0.1 volts can be a big change in many of these sensors.

 

The major sensors include a gear with missing teeth (reluctor) and a magnetic pick up coil that tell the ECU the angle of the crank, together these are called aptly enough, the crank angle sensor. Most of these sensors use Hall Effect or magnetism to count the teeth and reset the count. The more teeth, the more accurate the system can be due to more resolution. 32 and 64 tooth gears are common and can have either one or two teeth missing so that the ECU knows exactly where top dead center (TDC) is located for a given cylinder – usually cylinder #1 – and for each of the remaining cylinders. This sensor also lets the ECU know the rpm the engine is turning.

 

The next major sensor is the throttle position sensor. This is usually a round dash pot that rotates 90 degrees and gives the corresponding percentage of throttle opening.

 

Using these two sensors, a basic map can be made. The map is a 2 dimensional plot or table of throttle positions (or loads) running up one side, and rpm running along the other. This allows each engine to have a table for each throttle position at each rpm point. Each point on this plot is given an injector pulse unit (IJPU) to tell the ECU how long to keep the injector open at this rpm and throttle position. Plotting the correct air/fuel ratio and IJPU for each point is called mapping. The ignition timing is also plotted and mapped in the same way.

 

Once these plots are correctly mapped, the ECU has a reference to look up for each rpm and throttle position. But then atmospheric changes alter the correct mapping. So an oxygen sensor (or Lambda sensor for more exotic systems) is used to read the amount of unburned oxygen left in the exhaust. This sensor can be mapped and tabled so that the ECU knows the air fuel ratio that is actually happening in the engine and can offset the fuel map to meet a specific air/fuel ratio.

 

In some systems a mass air flow or MAF sensor is used to meter the air entering the engine and its temperature. And another sensor called a MAP or Manifold Air Pressure sensor is used to give the atmospheric pressure in the intake. Both of these sensors add to the accuracy of the ECU's calibrations and how well it responds to changes in ambient air, throttle position, and load. As sensors are added, the maps can be made more accurate and given additional dimension or reference tables.

 

Many systems today also use various designs of knock sensors to determine the onset of irregular combustion and reduce ignition timing to reduce or stop it from happening. Most designs are nothing more than a microphone that listens for sounds in the correct frequency. The best of these designs can use the resistance across the sparkplug gap to determine changes in chamber pressure and detect pre-ignition and detonation in their early stages. This allows these ECU's to alter timing in only the affected cylinders.

 

Other sensors include coolant temperature, gear, oil pressure, and the list goes on and on. These sensors allow the ECU to respond better to changes and input. However, many fast accelerating cars can actually out run the sensors like the oxygen sensor, and as a result, the base mapping becomes even more critical under hard acceleration.

 

On cars with traction control, the ECU uses sensors on the driveshaft and/or wheel hubs to determine tire slip, and then uses the brakes, and fuel and ignition timing to reduce power to the wheels and let the car hook up the power made.

 

The ECU can also control many other aspects of the drive train such as shift points for the trans, turning on and off cooling fans, and many other aspects up to and including the door locks when the car starts moving.

 

Some of the more modern engine management systems can determine the driving style of the driver and will adapt to, and adjust for, this driving style for best performance.

 

If each cylinder is fed fuel from one injector at a time, the system is called sequential injection. If it sprays the cylinders in groups, it is called batch fired - 2 at a time is called batch pairs, and a V engine design that sprays half of the engine at a time is called bank seperated. The difference in a well tuned sequential system and a batch system can be up to 3-4% more average torque.

If the engine sprays fuel near the valves, it is termed port injected. And systems that spray fuel with injectors near the throttle blade are termed throttle body injection. Some systems even do both.

 

 

How injectors work:

 

The injectors will flow a given amount of fuel at a given pressure. This flow is usually rated in cc per minute or pounds per hour. The amount of time the injector is open compared to how long it is closed is called duty cycle. The injector is just a simple electrical servo that opens and closes using electromagnets – just like the servo that locks and unlocks the doors on many cars. The injector can only be kept open for a maximum of 80% of its duty cycle or the servo will overheat and fail. The IJPU in the ECU's map tells the injectors when to open. The basic injector timing (when to close) is set based on valve closing events and other factors.

 

In many stock systems, the injectors are sized large enough to allow for additional flow if modifications are made to the engine for increased airflow. [Remember, if air is added to the engine, more fuel must be added in order to make more power.] But at a given point, the injectors will reach a limit to their duty cycle and no more flow can be had. At this point fuel pressure can be raised to increase flow. But this is not as straight forward as it seems. As fuel pressure is increased, the plunger in the injector takes longer to open and close as it fights the added pressure. At some point with increased power modifications, the injectors will be over run and larger flowing injectors will be needed

 

Eventually as you increase power over stock, at some given point, the fuel pump will not be able to flow additional fuel – especially at raised fuel pressures. At this point a larger, or additional, fuel pump and fuel lines are needed to keep up with additional power.

 

 

 

Tuning:

 

Stock vehicles are tuned from the factory to meet many different issues like emissions, MPG regulations, octane limits and fuel changes, load and use, etc. This results with many vehicles not being tuned for maximum power from the factory.

 

Aftermarket tuning companies use dynamometers and accurate air/fuel ratio testing to alter many vehicle's fuel and timing maps (and other ECU controlled aspects) for better performance. Generally different maps are devised for different octane fuels as well. These maps are then placed in a hand help programmer or flashing unit and sold to customers for their specific vehicle. Some of these units also allow you access and adjust many things in the ECU such as rpm limits, speedometer calibration, shift points for auto trans vehicles, throttle tip in (response) for drive-by-wire throttles, etc. But these maps are still mapped fairly safe to meet a wide variety of driving styles and fuels. Taking your car to a tuner with specific knowledge of your model and tuning experience can usually get you even more power. Dyno and tuning time with a respected tuner is not cheap, but power per dollar spent makes it a very worthwhile modification on many vehicles.

 

On vehicles with forced induction like turbos and superchargers, aftermarket tuning can make a substantial increase over stock power and can even improve mileage. And each time you increase boost, the tuning will need to be altered to match the additional airflow. But many pulley kit manufacturers will include or offer altered tuning as well.

 

 

 

Again, this is just a fairly brief overview, but I hope it helps others to better understand their cars.

[sharon]

Thanks that some good information.

[DVS2XS]

Thanks Sharon. I am no guru, but I have lots of experience with OEM, aftermarket, and custom designed systems. And I feel that others may want to know more.