TOPIC: Diesel Engine Modifications

As we continue our efforts to expand our fuel efficiency technologies to more and more engines, I am beginning to look at the science behind the fuel management and engine operations of the average Diesel engine.

I dug this up on GM diesels, but believe most of this information will apply to most (if not all) Diesel engines. I present this here as an educational aid for those wishing to learn more about these systems.

INTRODUCTION
This article covers basic description and operation of engine performance-related systems and components.
Read this article before diagnosing vehicles or systems with which you are not completely familiar.

AIR INDUCTION SYSTEMS > AIRFLOW SENSING > SPEED DENSITY >

All engines are equipped with a Manifold Absolute Pressure (MAP) sensor (gasoline), or Barometric Pressure (BARO) sensor (diesel), and use the speed density method to compute airflow rate. PCM uses manifold pressure to calculate the airflow rate. The MAP/BARO sensor responds to manifold vacuum changes due to engine load and speed changes. The PCM sends a voltage signal to the MAP/BARO sensor. Manifold pressure changes result in resistance changes in the MAP/BARO sensor.

By monitoring MAP/BARO sensor signal voltage, the PCM determines manifold pressure. On gasoline engines, if MAP sensor fails, the PCM supplies a fixed MAP sensor value, and uses the TP sensor to control fuel.

Some models also use an Intake Air Temperature (IAT) sensor. The IAT sensor allows PCM to determine intake air temperature. PCM uses signal to delay EGR until intake air temperature reaches about 40°F (5°C). If intake air temperature becomes excessively high, PCM compensates by slightly retarding timing.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > POWERTRAIN CONTROL
MODULE > MEMORIES >

PCM uses 3 types of memory:
Programmable Read Only Memory (PROM)PROM is factory programmed engine calibration data which "tailors" PCM for specific transmission, engine, emission, vehicle weight and rear axle ratio application. PROM can be removed from PCM. If battery voltage is removed, PROM information is retained.

Random Access Memory (RAM)RAM is the scratch pad for the CPU. Data input, diagnostic codes and results of calculations are constantly updated and temporarily stored in RAM. If battery voltage is removed from PCM, all information stored in RAM is lost.

Read Only Memory (ROM)ROM is programmed information which only PCM can read. ROM program
cannot be changed. If battery voltage is removed, ROM information is retained.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > INPUT DEVICES >
ACCELERATOR PEDAL POSITION SENSOR >

The Accelerator Pedal Position (APP) sensor, mounted on accelerator pedal, contains 3 separate variable
resistor circuits that monitor throttle opening angle for PCM. Each APP circuit, connected to a 5-volt reference signal, has a high resistance value when throttle is closed. At wide open throttle, sensor resistance value is low and output to PCM will be about 5 volts.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > INPUT DEVICES >
BAROMETRIC ABSOLUTE PRESSURE SENSOR >

The Barometric Absolute Pressure (BARO) sensor is part of MAP sensor mounted on left side of cowl, and monitors atmospheric pressure during ignition key-on, engine off. The signal is converted into an altitude value by the PCM. PCM uses this information to adjust fuel flow, injector timing and transmission shifting.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > INPUT DEVICES >
CRANKSHAFT POSITION SENSOR (57X SIGNAL) >

The Crankshaft Position (CKP) sensor is a Hall Effect type sensor mounted at front of crankshaft sprocket. Front crankshaft sprocket has 57 teeth and plus a sync gap. CKP sensor will output and on-off pulse as each window passes the sensing element. CKP sensor is connected directly to PCM by a 12-volt reference circuit, a low reference circuit and a CKP sensor signal circuit. PCM uses CKP sensor signal to determine engine RPM. This signal is used to improve idle. If pump cam signal is lost, PCM will use crankshaft position signal data to control injection timing and fuel flow.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > INPUT DEVICES > FUEL
TEMPERATURE SENSOR >

Sensor is part of pump cam signal sensor and works like IAT sensor. PCM uses this signal to adjust fuel delivery.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > INPUT DEVICES > INJECTION PUMP CAM SIGNAL >

The injection pump cam signal is an optical sensor mounted on the fuel injection pump. The sensor receives a 5-volt reference signal and allows the PCM to measure fuel injector pulse ring RPM and position. This signal is critical to accurate fuel injection timing and start of injection.

COMPUTERIZED ENGINE CONTROLS (DIESEL ENGINES) > INPUT DEVICES > INTAKE
AIR TEMPERATURE SENSOR >

The Intake Air Temperature (IAT) sensor is a thermistor (temperature sensitive resistor). The IAT sensor
measures the temperature of the air entering the engine. The PCM supplies 5 volts to the IAT signal circuit.

When the IAT sensor is cold, the sensor resistance is high. When the air temperature increases, the sensor
resistance lowers. With high sensor resistance, the PCM detects a high voltage on the IAT signal circuit.

With lower sensor resistance, the PCM detects a lower voltage on the IAT signal circuit. If the PCM detects an excessively high or low IAT signal voltage, a DTC will set.

FUEL SYSTEMS (DIESEL ENGINES) > FUEL DELIVERY > FUEL INJECTION PUMP >

The fuel injection pump is a mechanical high pressure pump. The fuel injection pump is located below the intake manifold. Fuel is pumped to the fuel rails at a specified pressure. Fuel pressure is regulated by a valve on the inlet of the fuel pump, controlled by the ECM. Excess fuel from the fuel injection pump returns to the fuel tank through the fuel return pipe and a fuel cooler.

On vehicles equipped with dual fuel tanks, an electric transfer fuel pump is located on the left frame rail. This fuel pump is powered by the fuel pump relay that is controlled by the ECM. Fuel is transferred from auxiliary fuel tank to primary fuel tank in order to ensure all of the usable fuel volume is available to the fuel injection pump.

FUEL SYSTEMS (DIESEL ENGINES) > FUEL CONTROL >
The ECM, using input signals, determines adjustments to the air/fuel mixture to provide the optimum ratio for proper combustion under all operating conditions. Fuel control systems can operate in the open loop or closed loop mode.