INSTANT EXPERT
Of ten overlooked, glow plugs are vital not just to get a diesel engine going but also to keep it running happily, as Rob Marshall discovers.
Compression engines’ advantages over those that employ spark ignition include lower CO2 emissions and greater efficienc y, using cheaper fuel. Sadly, the political tide has turned against the diesel motorcar. While UK taxation policy has made diesel more expensive, comparable petrol engines struggle to better diesel’s miles-per-gallon figures. Even when relying on expensive and complicated means of high-voltage hybrid electric drive, diesels tend to have the fuel efficiency advantage on long , high-speed journeys. Although new diesel engine demand has dropped considerably, due mainly to changes in government fiscal policy for business f leets, diesel cars remain on British roads in significant numbers, meaning that repair demand is at an all-time high. Appreciating how the modern glow plug circuit works and why it must perform in cer tain ways is essential to achieving long and economical life from your diesel vehicle.
sWhy are glow plugs necessary?
Diesel engines work by sucking in and compressing air to 30-55 BAR to attain sufficiently high temperatures (700-900°C) in which to burn a heavy oil fuel, when injected into the combustion chamber. While this principle works well with a warm engine, the air may not get sufficiently hot on a cold morning. Therefore, some kind of pre- warming is necessar y.
Clearly, the naked f lame heaters of stand-alone industrial engines were inappropriate for motor vehicles. Electricity provided the answer. By inserting a coiled heating element into the cylinder, heat could be introduced before the engine was cranked. Yet, as each cylinder requires a dedicated glow plug, plus a more powerful starter motor to overcome the higher compression ratios, diesel engines require larger, heavier and more expensive batteries. The time delay poses an inconvenience, too. Due to the driver having to wait for the glow plugs to warm, a cold start could not be instantaneous and tended to be followed by disconcer ting clattering. Yet, modern engine refinements, including those made to glow plugs and their control units, have mitigated these disadvantages, helping to explain why diesel engines became so popular. As they have become more sophisticated, glow plug roles have also altered, which shall be detailed later.
What are glow plugs?
Glow plugs are the hardest- w
