Diesel fuel evaporates much slower than gasoline and requires more heat to cause combustion in the cylinder of the engine. For this reason, preheating devices and starting aids are used on diesel engines. These devices and starting aids either heat the air before it is drawn into the cylinder or allow combustion at a lower temperature than during normal engine operation.
In cold weather, coolant heaters are used to keep the fuel flowing freely. There are three common types of heaters used on mobile diesel engines: immersion block heaters, circulating tank heaters, and fuel-fired heaters.
The immersion block heater is installed directly into the engine block in a location predetermined by the manufacturer. The warmed coolant circulates around the cylinders in the block by convection. The heater is powered by either 120 or 240 volts.
Circulating tank heaters are installed in a way that creates circulation. The coolant leaves through the bottom of the engine block and travels to the heater. The heated coolant rises and is transferred back to the top of the engine block. These types of heaters should be of a higher wattage than immersion block heaters because some of the heat is lost through the hoses and tank. Circulating heaters are available in 120 or 240 volts.
Fuel-fired heaters are used to heat the engine and cab unit via the coolant by burning diesel fuel. These units burn less fuel than the engine would if left idling. The unit operates like a kerosene-fired space heater. The flame warms the coolant that is flowed through the space.
Most oil heaters are electric-powered immersion heaters that are installed in the oil sump through the drain plug or the dipstick opening. A thermostat can also be installed as part of the heating unit. This type of heater is designed to keep the oil pan warm; the heat radiates upward to warm the entire engine.
The purpose of a glow plug is to heat up the air that is drawn into the precombustion chamber to assist starting, especially in cold weather. Operating temperatures of 1500ºF can be reached in a matter of seconds. Glow plugs are common on precombustion chamber engines, but not on direct injection diesels because they use shaped piston crowns that produce a very effective turbulence to the air in the cylinder.
Direct injection engines also have less immediate heat loss to the surrounding cylinder area than in a precombustion engine and generally have a higher injection spray-in pressure.
A glow plug is used for each cylinder located just below the injection nozzle and threaded into the cylinder head (Figure 4-18). The inner tip of the glow plug extends into the precombustion chamber. The glow plugs may be turned on using the ignition switch with the length of time being controlled from an electronic module.
Figure 4-18 — Glow plugs.
During colder weather, the system may have to be cycled more than once to start the engine.
Glow plugs are not complicated and are easy to test. Disconnect the wire going to the glow plug and use a multimeter to read the ohms resistance of the glow plug. Specifications for different glow plugs vary according to the manufacturer. Be sure and check the manufacturer’s service manual for the correct ohms resistance value.
Starting fluids must never be used if the engine is equipped with either glow plugs or an electric intake heater.
Ether is a highly volatile fluid that is injected into the intake manifold as you crank the engine. It is found in an aerosol or capsule-type container. Since ether has a low ignition point, the heat generated in the combustion chamber is able to ignite it. Heat from this ignition then ignites the diesel fuel, and normal combustion takes place. Once the diesel engine starts, no more fluid is required. Use of spray cans of ether is discouraged. The only method of safely using ether is with a closed dispensing system.
Automatic ether systems are wired into the cranking circuit and dispense starting fluid when the cranking circuit is energized. Flow of the ether is controlled by a valve orifice or rapid cycling of the valve, resulting in continuous atomizer flow. An engine temperature switch prevents operation when the engine is warm.
The nozzle directs the flow of starting fluid into the airstream and determines the rate at which it is injected. The direction of the spray should be against the flow of incoming air to maximize mixing with the air.
Cold starting aids such as ether should be used only in extreme emergencies. Too much ether may detonate in the cylinder too far before top dead center (BDTC) on the compression stroke. This could cause serious damage, such as broken rings, ring lands, pistons, or even cracked cylinder heads. If you must use ether, the engine has to be turning over before you spray it into the intake manifold.