Control Principle And Maintenance of Diesel Engine Preheater

Many vehicles equip diesel engines. They have the large load capacity and run in harsh environments. Many drivers work in mountainous or alpine areas for years. Their vehicles have various devices to improve cold starting performance. Let's talk about the control principle and maintenance of the diesel engine preheater.

1. Function

2. Working principle

3. Main structure

4. Preheat indicator

5. Control of preheating time

1. Function

Diesel engines are compression ignition engines. There is no spark plug. During the diesel engine's cold start, even if the cylinder air is fully compressed. Part of the compression pressure will leak from the combustion chamber.

At the same time, the crankshaft speed is slow when starting, and diesel fuel is easy to condense. In the cold start, the mixture in the cylinder is difficult to warm up to the self-ignition temperature. It needs a preheater to increase the mixture temperature.

Most diesel engine preheaters warm up the air drawn in. There are two types of preheater: one is the flame jet type, and the other is the electric wire type. The former uses fuel combustion to heat the incoming air, while the latter uses a resistance wire to heat the incoming air. 

The flame jet preheater is mounted on the air intake pipe. Common direct injection diesel engines are mounted in the combustion chamber (or vortex chamber), and high power direct injection diesel engines are generally mounted on the air intake pipe.

2. Working principle

Some light diesel engines' cold start preheating device is flame injecting type. The device consists of a preheating controller, water temperature sensor, preheating indicator, flame injector and fuel solenoid valve.

The flame jet preheater has a tubular valve with a large linear expansion coefficient. It has a cavity inside the tube and an fuel pipe fitting at the top. 

It connects to a small fuel tank through the fuel pipe. The lower part is cemented together by threads with the spool. The top of the spool encloses the fuel hole.

The nickel-chromium electric wire is wound on the outside of the valve. One end of the wire is connected to the housing and lapped. The other end is connected to the terminal block.

When the preheater is not working, the conical tip of the spool closes the small oil hole under the effect of the cold shrinkage.

When the diesel engine starts, the starter switch is placed in the preheating position. The circuit of the electric heating wire is connected. The valve is heated and elongated so that the valve spool moves down and the fuel hole opens.

The fuel is ejected from the lower end of the valve body and vaporized, which meets the high-temperature electric heating wire and forms a flame. The flame heats the air entering the cylinder. To make the flame more stable, the vehicle is equipped with a stable flame cover.

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3. Main structure

The electric heating wire type preheater is also called a diesel glow plug. It is screwed in the cylinder head. The lower end is the electric heating wire. It extends into the cylinder of the vortex chamber or the air tube.

The diesel glow plug consists of a spiral tubular iron-nickel-chromium electric heating wire, a heat-resistant stainless steel heating sleeve, a central conductive tube, an insulating porcelain tube, a housing and other components.

The internal circuit of the electric preheater is the central conductive tube to the heating wire to the heating steel jacket to the housing to the ground. The electric heater wire is connected in parallel or in series, usually in parallel.

The working current of the electric preheater is 2 to 6A. The temperature can rise to 900°C. The normal service life is more than 1000 times.

Delcoribo Diesel Glow Plugs

Some engines are equipped with self-regulating temperature devices. The diesel glow plug is fitted with a control coil that increases in resistance as the temperature rises. It relies on the increased resistance of the coil to reduce the electricity to the hot coil (in series with the control coil). As the electric flow is restricted, the temperature of the preheat plug does not rise too high.

In addition, there is a kind of "electric preheating net". It makes electric heating wires into a net, which is fixed on the outer frame of the piece. The shape and size of the outer frame are the same as that of the air inlet of the air intake manifold. 

The electric preheating net is installed on the air intake manifold's air inlet. When the diesel engine starts, the preheating net is energized and heated so that the air passing through the electric preheating net is heated, which can improve the starting performance.

4. Preheat indicator

The preheat indicator is installed to monitor whether the preheater circuit is on and functioning. If the preheat indicator does not stay on, the preheater is not working, or the indicator is damaged.

The preheat indicator is on the dashboard. When the light stays on, it indicates that the "engine is ready to warm up". When the engine starts, the indicator goes out.

The indicator light flashes for 1 min or less. In some cases, the working time of the indicator can be varied depending on the coolant temperature.

5. Control of preheating time

Take one Toyota as an example. When the diesel engine starts at a low temperature, the preheat timer and timers 1 and 2 inside the emission control ECU are switched on. Timer 1 switches on the preheat indicator. Timer 2 switches on the preheat plug relay to generate heat.

During the warm-up time, which is determined by the coolant temperature, timers 1 and 2 are both switched on and then off simultaneously. When timer 1 is disconnected, the preheat indicator light also goes out.

When the ignition switch is turned to the "Start" position, the preheat timer or emission control ECU switches the glow plug relay on to prevent the temperature of the preheat plug from dropping at start-up.

When timer 3 is working, the preheat relay is switched on for a time determined by the coolant temperature, thus heating the preheat plug. When the ignition switch is turned from the "Start" position to the "ON" position, it provides an auxiliary function for starting the engine.

There are two main forms for the system to control the preheating time.

①  Fixed delay type

In this type of system, a preheat timer is installed to control the time for which the preheat plug relay is energized (preheat time). During this time, the indicator lights up for 5s, and the preheat for 18s. Both of these are controlled at a fixed time.

②  Variable delay type

In this type of system, the preheat timer randomly controls the time for the preheat plug relay to energize according to the coolant temperature and the alternator's output voltage. The preheat indicator stays on for 2 to 28s, and the preheat time is 2 to 55s.

Obviously, the second one is better. This method is related to the coolant temperature. The temperature is very low in winter. In the case of starting the engine, you should first put the start switch in the "1" position so that the preheater circuit turns on.

The engine starts to heat the intake air. Then put the start switch in the "2" position. After that, the engine can be started smoothly.

The engine preheater has high power consumption (as evidenced by the light intensity change in interior lighting when the preheater relay is switched on and off). The preheater is working on a short-time basis.

If the diesel engine does not start several times, it should be temporarily stopped. The reason for the difficulty in starting should be identified. The preheater should not be connected for a long time. Otherwise, it may be burned.

If the preheater does not start for the first time, stop for 10-20s, and preheat for another 0.5min. If it still does not work, it should be repaired.

The correct tightening torque should be used to install the preheaters. For Toyota diesel engines, the socket spanner with a depth of 12 mm is used to remove and install four preheaters with a tightening torque of 13 N-m.

When cleaning the preheater, you should avoid getting oil or diesel fuel on it. You can use a dry cloth to wipe the oil off the preheater terminals and the rubberwood pad.

Testing the performance of the preheater requires that it is carried out quickly and accurately. When the preheater has burned red, it should be disconnected from the power supply and not be energized for too long to prevent damage to the engine.

Take care when replacing the preheater not to damage the casing and flame stabilizer cover. Otherwise, it causes a broken circuit and shortens the service life of the preheater.

The working condition of the flame jet preheater should be checked regularly. When the power is turned on, the electric heating wire should heat up to red and have a flame emitted within 0.5 min. If no fuel is coming, don't use it. Otherwise, the electric heating wire will be burnt.

It has been investigated that many preheater damages are caused by the fuel pipe being blocked. The driver turns on the power switch for a long time. It is recommended to use the fuel pump handle before the preheater switch is turned on.

Lauritz Carolsfeld        January 13, 2022
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