Diesel Engine Preheating Devices
Figure 1: Electric intake air heater
Diesel engines use the principle of self-ignition, where fuel is ignited by the heat generated during air compression. However, during cold starts, the temperature increase from compression may be insufficient, preventing fuel ignition. In such cases, preheating devices are used to assist the temperature increase from compression and facilitate engine starting.
These preheating devices include systems that preheat intake air using air heaters installed in the intake air passage or systems that increase the temperature inside the combustion chamber using glow plugs installed in the chamber.
Generally, air heaters are used in medium and large vehicles, while glow plugs are often used in small vehicles.
Table of Contents
Electric Intake Air Heater Preheating System
Figure 2: Electric air heater circuit diagram
The electric intake air heater operates by passing current through an air heater installed in the intake passage, as shown in Figure 1-2, causing it to glow red-hot and preheat the intake air.
Figure 2 shows the circuit diagram of the electric air heater, which consists of components such as the air heater, ECU, heater relay, indicator lamp, water temperature sensor, and the air heater itself.
The presence of a water temperature sensor indicates that this intake air heater features an automatic control function, where the ECU calculates and adjusts the preheating time based on the engine coolant temperature during startup to preheat the intake air to an optimal temperature.
Figure 2-1: Electric air heater circuit diagram
The operation is explained based on the diagram above.
- When the key switch is turned ON, the ECU determines whether the coolant temperature (hereafter referred to as preheating water temperature) is within the range requiring preheating based on input signals from the water temperature sensor.
- If within the preheating water temperature range, the ECU energizes the heater relay, turning it ON, and simultaneously lights up the indicator lamp. This allows current from the battery to flow to the air heater, initiating preheating.
- The ECU continues preheating for a duration corresponding to the preheating water temperature and stops energizing the heater relay once the specified duration is reached, turning the relay OFF.
- When the heater relay turns OFF, the indicator lamp also turns off.
- After the indicator lamp turns off, turning the key switch to START activates the starter, starting the engine.
- The air heated by the air heater is drawn into the cylinder, ensuring adequate compression heat even during cold conditions, making engine starting easier.
Additionally, some electric intake air heaters are equipped with mechanisms to continue preheating even after engine startup to ensure smooth engine operation.
Glow Plug Preheating System
Glow plug preheating systems are used in diesel engines, particularly in swirl chamber engines and some direct injection engines.
Based on preheating time (duration of current flow to the glow plug), they are classified into standard and rapid types.
The glow plug preheating system consists of components such as the ECU, glow plug, glow plug relay, indicator lamp, and water temperature sensor.
Figure 3: Glow plug temperature rise characteristics
Figure 3 shows that the temperature rise characteristics of preheating systems are divided into standard and rapid types. Among glow plug types, self-temperature-controlled glow plugs are classified as rapid type.
Self-temperature-controlled glow plug preheating systems are classified as rapid type, which will be explained later.
Standard Glow Plug Preheating System
Figure 4: Glow plug installation position
The standard glow plug preheating system consists of glow plugs installed in the combustion chamber of the cylinder head, as shown in Figure 4, a glow plug pilot for confirming the red-hot state of the glow plugs, and a glow plug relay that controls the current flowing through these components.
Standard Glow Plug
Figure 5: Normal glow plug
The standard glow plug, as shown in Figure 5, is a device where a heat coil is embedded within a protective metal tube.
The gap between the heat coil and the metal tube is filled with heat-resistant insulating powder, which serves to insulate and hold the heat coil in place.
Glow Plug Pilot
The glow plug pilot is an indicator lamp used to visually confirm the red-hot state of the glow plugs. It is typically controlled by the glow plug relay and lights up while the glow plugs are appropriately red-hot, turning off once preheating is complete.
Figure 6: Glow plug pilot
Figure 6 shows the glow plug pilot, which consists of a heat coil, supporting terminals, and a protective cover. It glows red along with the glow plugs due to the current flow, indicating the preheating state (glow lamp status).
This indicator lamp type is actually quite old, and was installed in the driver’s seat of Old car, serving to inform the driver of the preheating status andproviding important information for Engine start timing.
Figure 6-1: Glow plug pilot indicator
In current diesel vehicles, the glow plug pilot is commonly displayed as an LED lamp in the form of a mark, as shown in Figure 6-1, within the meter panel, offering longer life and lower power consumption.
This glow plug pilot indicator is installed as one of the warning lights in the driver’s seat meter panel and is used to indicate the red-hot state of the glow plugs.
When the key switch is turned ON, the glow plug relay operates, and the glow plug pilot lights up for a while (the duration depends on the engine’s condition, but typically around 2-3 seconds). Once the preheating by the glow plugs in the engine is complete, the glow plug pilot turns off.
Turning the engine key at this timing activates the starter, and the engine starts.
In current vehicles, this process is integrated, and by simply pressing the engine button, the preheating of the glow plugs and the operation of the starter are automatically performed.
Glow Plug Relay
Figure 7: Glow plug relay
Figure 7 shows that the glow plug relay consists of two independent relays, one for preheating and one for starting, housed within a single case, each activated by the operation of the key switch.
The voltage applied across the glow plugs during preheating remains unchanged even when the starter is engaged, ensuring that the glow plugs reach the appropriate temperature.
Figure 8: Normal glow plug preheating device circuit
Figure 8 explains the operation of the standard glow plug preheating system.
- When the key switch is turned to the “START” position, current flows from the battery through the glow plug relay from terminal g to terminal E, activating the relay. This closes contact P1, allowing current to flow to the glow plugs through the glow plug pilot, initiating preheating.
- After confirming the heating state of the glow plugs by the red-hot glow plug pilot, the key switch is turned from “START” to the starting position (turning the key from START). This disconnects the voltage between the glow plug relay’s terminals g and E, opening contact P1. Simultaneously, current flows from terminal ST to terminal E, closing contact P2.
- With contact P2 closed, the current to the glow plugs bypasses the glow plug pilot and flows directly from the battery. At this point, the starter also operates, but the red-hot state of the glow plugs is maintained.
After the engine starts, returning the key switch to the “OFF” position cuts off the current to the starter and glow plugs, ending the operation.
Self-Temperature-Controlled Glow Plug Preheating System
Figure 9: Self-temperature controlled glow plug
Metal Type
Figure 9-1: Self-temperature controlled glow plug (metal type)
The metal type self-temperature-controlled glow plug, as shown in Figure 9-1, is designed with a protective metal tube on the outside. Inside, it has a rush coil that acts as a heat coil and a brake coil connected in series, which increases resistance and reduces current as the temperature rises. This design allows the glow plug to maintain a specified temperature with self-temperature control.
Rush Coil and Brake Coil in Series
By connecting the rush coil and brake coil in series, the following operation is achieved:
- Rush Coil
- Brake Coil
- Effect of Series Connection
The rush coil has the characteristic of increasing resistance as the temperature rises.
The brake coil controls the amount of current, smoothing out the changes in current due to temperature rise, thus improving the stability of the entire system.
The series connection of the brake coil and rush coil results in the following actions:
- In the initial state, the heat coil carries the current and starts to heat up rapidly.
- As the temperature rises, the resistance of the rush coil increases, reducing the current.
- The brake coil mitigates the change in current, suppressing transient current fluctuations.
- Consequently, the temperature of the entire system is maintained at the specified value, preventing overheating and ensuring stable operation.
In the initial state, the heat coil passes a current through it and heats up rapidly. As the temperature rises, the resistance of the heat coil increases and the amount of current flow decreases.
As a result, the temperature of the glow plug is maintained at a specified value, preventing overheating.
This mechanism enables self-temperature regulated glow plugs to operate efficiently and safely.
Ceramic Type
Figure 9-2: Self-temperature controlled glow plug (ceramic type)
As shown in Figure 9-2, the ceramic type self-temperature-controlled glow plug consists of a heating element (conductive ceramic) and an insulator (insulating ceramic). It also has a control coil connected in series, which increases resistance and reduces current as the temperature rises, allowing the glow plug to maintain a specified temperature with self-temperature control.
The operation of the self-temperature-controlled glow plug preheating circuit is explained in the following Figure 10.
Figure 10: Preheating circuit of self-temperature controlled glow plug preheating device
The following explains the operation of the preheating circuit of the self-temperature-controlled glow plug system as shown in Figure 10.
- When the key switch is turned to the ON position, the indicator lamp lights up, and current flows to the ECU. The ECU, based on the input signal from the water temperature sensor, determines whether the coolant temperature is within the preheating range. If it is, the ECU energizes the glow plug relay, turning it ON, and current flows from the battery to the glow plugs, starting the preheating.
- The ECU continues to energize the glow plug relay for a duration suitable for the coolant temperature, keeping the glow plugs heated. When the system indicates that it is ready for starting, the indicator lamp turns off, notifying the driver.
- With the key switch in this position, turning it to START engages the starter, and as the engine begins to rotate, the air heated by the glow plugs gets compressed, raising the compression heat and facilitating engine start.
- After the engine starts, the ECU keeps the glow plug relay ON for a certain period, depending on the coolant temperature, ensuring the glow plugs maintain the necessary temperature for smooth engine operation.
References
This article was written with reference to and citations from the following books: