When diagnosing and repairing today’s modern diesel engines, you often trust your scan tool to give you answers. But what do you do when it gives you more questions? We decided to investigate a mystery that has always eluded us until now.
As diesel engines have become more electronically controlled, I have been baffled why the injection rate that is typically displayed on the scan tool is given in mm3 (millimeters cubed). When working with a gasoline engine, the injection rate is often based on m/s (milliseconds). This refers to how much time the injector is commanded to open based upon inputs to the Powertrain Control Module (PCM).
Fuel pressure is supplied to each injector and depending on the engine’s running conditions will be pulsed for a period of time in milliseconds to achieve the proper air/fuel ratio, which it reads from the oxygen sensor. So why don’t modern diesel engines use m/s instead of mm3?
Similarly, when diagnosing different diesel engines with a scan tool, you can usually display a diesel injection event in m/s, which seems logical. The main reason the injection rate of a diesel engine would be displayed as mm3 is to illustrate fuel flow rate. This is the precise amount of fuel that’s needed for that injection stroke to maintain the demands of the engine.
To better explain this, it’s the amount of fuel that will be injected into the engine through the injector’s nozzle. One mm3 is equal to .001/cc. Therefore, when looking at the data on a scan tool, you may find, for example, that the main injection rate at idle is 8.4 mm3. This can then be converted to .0084 cc’s of fuel that the engine needs in order to run properly at that operating parameter.
These terms may not mean much at first glance, but think of it this way: The PCM in a diesel engine is precisely programmed to know exactly how much fuel is needed to maintain the power that is commanded by the operator (aka, your right foot). Instead of basing injection events related to time in milliseconds, the PCM is basing fuel demands of the engine on the flow rate of the injector.
Another important point to consider is timing the injection event. The diesel injector is pulsed by the PCM for a given time to obtain the correct fuel flow rate, whereas in gasoline engines, the fuel flow rate is not emphasized.
Diesel injection events are often found in microseconds (µs). A microsecond is the equivalent of 1/1,000,000th of a second. A millisecond is 1/1,000th of a second. Let’s go back to our example of the main injection rate of 8.4 mm3. For the injector to flow a rate of .0084 cc’s, the PCM will energize the injector for 439 µs. We can convert this to .439 milliseconds.
Of course, there are parameters of the engine that the PCM monitors in order to know what the injection rate must be. These are no different than the parameters found in a gas engine and include such things as engine temperature, throttle position, barometric pressure, manifold pressure, ambient temperature, oil temperature, etc.
Electronic diesel engines are often operated in an “open loop” mode. In gasoline engines, open loop is seen when the engine is operated on a set of tables programmed into the PCM/ECU based on inputs from its oxygen sensors. In open loop mode, the ECU pulls from the map and not the sensors, which is used to help trim the fuel curve for more efficient engine operation. When an oxygen sensor is used to run the engine it is called, “closed loop.”
Today’s diesel engines can also run in closed loop operation even though there are no oxygen sensors. There are, however, other sensors that are incorporated such as crank angle sensors and cam sensors, which help trim fuel based on combustion spikes of the crankshaft.
One thing that we know for sure: The technological era for diesel engines is not anywhere near slowing down. These engines will only become more in-depth with electronic devices as long as the emissions regulations and demands continue to increase.
Diesel injection concerns more than just fuel flow rate, however. Depending on the amount of diesel emission devices used, there may be as many as five injection cycles per combustion stroke in a common rail system. These injection strokes consist of pilot, main, and post injection events.
The pilot injection event happens when a little bit of fuel is injected into the cylinder as the piston is traveling up the bore and compressing air. This small amount of fuel excites the charge eliminating the “cold burn,” which will light off the main injection event. This helps a diesel engine run quieter and burn the main injection more efficiently.
There may be as many as three main injection strokes commanded by the PCM, which is dependent on engine speed and the emission devices that may be actuated at that time. By continually striving for cleaner emissions, the side effects have brought about the reduction of soot and NOx, so several main injection events at different times may be necessary in order meet the ambitious goals we have set.
The post injection event is used for devices such as the DPF (Diesel Particulate Filter), and occurs when fuel is injected as the exhaust valve is open to inject fuel into the exhaust mounted DPF system. The DPF unit is more or less a big screen, which is used to capture soot. As the soot collects in the screen, at some point it will have to be burned off to regenerate itself.
The fuel introduced during the post injection cycle lights off in the DPF, cleaning the soot from the unit. Regardless of the injection event, the fuel rate for each one is precisely calibrated in the PCM for optimum engine control by using a fuel flow rate of mm3.
Even though diesel engines have become more computerized, the power and efficiency gains have also increased. Common rail injection has improved power by as much as 40 percent and efficiency by at least 30 percent. So by having injection rates precisely timed and monitored, it has opened another page: making more power on less fuel with cleaner air.