Diesels are up to 30% more efficient than gasoline engines because they use high compression to ignite the fuel rather than spark plugs. High-pressure fuel injectors spray fuel directly into the combustion chambers when the pistons near top dead center. The heat of compression, combined with residual heat in the piston and cylinder head from the previous power stroke, ignites the fuel.
The only drawback with this approach is that a diesel can be hard to start when the engine is cold. Since there is no residual heat in a cold engine, some means of supplemental heat must be provided to make cold starting easier, especially when the outside temperature drops below freezing. Without this supplemental heat to get the fire going, the engine may not start at all. It will just crank and blow white smoke (unburned fuel vapor) out of the tailpipe.
The supplemental heat source for cold starting most passenger car and light truck diesel engines is the glow plug system. Some truck diesel engines use an electrically heated grid in the air intake system to provide start-up heat, but for now, glow plugs are still the primary aid for cold-starting small diesel engines.
VW GLOW PLUG SYSTEMS
The late-model Volkswagen 1.9L TDI “Turbo Direct Injection” diesel engine in the Jetta, New Beetle and Golf, and the 2.0L turbo diesel in the Passat models use a Bosch “Duraterm” fast-start type of glow system that takes only a few seconds to reach 850° C (nearly 1,600° F). Once the glow plugs reach the operating temperature, the engine can be cranked. While cranking, the glow plugs continue to heat and may reach 1,150° C (2,100° F) until the engine starts.
The major components in the glow plug system are the glow plugs (one per cylinder), wiring harness, automatic glow time control module (located in the cowl plenum area of the engine compartment on the driver’s side), glow plug relay (in the auxiliary relay box), glow plug indicator lamp (the icon looks like a sideways coil spring), engine coolant temperature sensor and the engine control module. The engine also has three additional glow plugs for warming up the coolant following a cold start, but these are not required for starting the engine.
When the ignition switch is turned on, the engine control module looks at the engine temperature to see if supplemental heat is needed to start the engine. If the engine was recently driven and is still hot, the glow plugs are not needed and are not turned on. But if the engine has cooled off sufficiently or is cold (typically below 9° C or 48° F), the engine computer turns on the glow plug relay and control module. The module then routes voltage to the glow plugs, causing them to rapidly heat up. On older VW diesel engines, just a relay was used to power on the glow plugs (no separate module).
The glow plugs are located in the cylinder head and the tips extend all the way into the combustion chambers in the TDI engine. This allows the heat to be quickly absorbed by the incoming air charge. In the older VW diesel engines with prechamber heads (1.9L and 1.6L), the glow plugs extended only into the prechambers and are just below the fuel injector. This setup takes more heat because much of the heat is absorbed by the prechamber itself. Eliminating the prechambers and extending the glow plugs into the combustion chamber significantly reduces the warm-up time, and also allows the glow plugs to continue to assist the ignition process after the engine starts and is warming up.
On the 1980s vintage VW diesels, the glow plugs were turned off once a predetermined starting temperature was reached. Starting in the 1990s, the glow plugs were kept on while the engine was cranking, and for a brief period after the engine began to run, to prevent the engine from stalling. A thermo time switch or the engine control module turned off the glow plug relay when the supplemental heat was no longer needed. On the latest generation of VW TDI diesel engines, the glow plugs remain on for up to four minutes after the engine starts to smooth the idle, and reduce noise and hydrocarbon emissions. Exhaust emissions are highest following a cold start, so keeping the glow plugs on during a “post-glow” period allows the engine to meet lower emission standards. In this respect, the glow plugs act almost like spark plugs to keep the fire lit until the engine’s temperature stabilizes and emissions taper off. If engine speed goes above 2,500 rpm during the warm-up period following a cold start, the computer will shut off the glow plugs. The latest VW glow plugs are made of more durable materials, and are designed for a longer service life. On the Bosch glow plugs, the tube that extends into the combustion chamber is made of Inconel 601 steel, which is highly corrosion resistant. A pair of high-temperature Viton seals are used to seal the tube and internal heating coil to the gas-tight tube, and a ceramic powder is used inside the tube to provide good thermal conductivity. The glow plugs are designed to last upward of 100,000 miles, and most do — but some do not.
