How a Diesel Engine Works: No Spark Plugs, DEF and Everything in Between

02 · Starting it up

Glow Plugs and Cold Starts

On a cold morning, the cold metal of the engine soaks heat out of the compressed air, and the air may not get hot enough to light the fuel. That’s where glow plugs come in. Each cylinder has one: a pencil-shaped heater that pokes into the combustion chamber and glows red-hot, over 1,000 °C in modern ceramic plugs, within seconds.

In older diesels you waited for a coil-shaped ‘wait to start’ light on the dash before cranking. Modern glow plugs heat so quickly that the wait is usually a second or two, and they keep glowing after the engine starts to cut smoke and noise while it warms up.

The rest of the starting system works like a gas engine’s, just beefier. A diesel needs a stronger starter to crank against all that compression, and many diesel trucks carry two batteries.

Glow plugs from a car diesel after 130,000 km. There is one per cylinder.

• Glow plugsThe cold-start heatersControlled by a module that decides how long to heat based on engine temperature. When one fails, you’ll notice harder cold starts and white smoke.
• Block heaterThe Canadian essentialAn electric element in the coolant that you plug into an outlet overnight. It keeps the engine warm, so cold starts are easier and cause less wear.
• Winter dieselFuel that won’t gelDiesel contains natural waxes that thicken and clog filters in deep cold. Stations in cold regions switch to winter blends, and additives can lower the gelling point further.

04 · Emissions and DEF

Soot, NOx and the Blue Fluid

Diesels burn lean and hot, which is great for efficiency but produces two pollutants in larger amounts than gas engines: soot (tiny carbon particles) and nitrogen oxides, or NOx. Modern diesels clean their exhaust with a chain of devices, which is why a new diesel pickup’s exhaust is far cleaner than one from the 1990s.

Cleaner fuel made this possible. Canada and the United States cut on-road diesel to 15 ppm sulfur in 2006, and Europe went to 10 ppm in 2009, because sulfur poisons the catalysts and filters.

A diesel particulate filter (right) beside an oxidation catalyst. The honeycomb traps soot.

The exhaust clean-up line, in order

• 1. EGRCools the fireExhaust gas recirculation feeds some exhaust back into the intake. Less oxygen means cooler combustion and less NOx, though the valve and cooler can clog with soot over time.
• 2. Oxidation catalystBurns off the easy stuffConverts carbon monoxide and unburned fuel into carbon dioxide and water, and heats the exhaust for the next stage.
• 3. Particulate filterTraps the sootA ceramic honeycomb with alternate channels blocked, so exhaust must pass through the walls. Trapped soot is burned off at high temperature in a process called regeneration.
• 4. DEF injectorAdds the blue fluidA dosing injector sprays diesel exhaust fluid into the hot exhaust, where it breaks down into ammonia.
• 5. SCR catalystRemoves the NOxOn the selective catalytic reduction catalyst, the ammonia reacts with NOx and turns it into harmless nitrogen and water vapour.
• 6. NOx sensorsKeep scoreSensors before and after the SCR check that it’s working. Faults trigger a warning and, if ignored, reduced power.

DEF: what every diesel owner should know

• 32.5%Urea, mixed with purified water
• −11 °CFreezing point (the tank is heated)
• 1.5–2 LUsed per 1,000 km in a light-duty diesel
• ISO 22241The quality standard on the label

DEF goes in its own tank, usually with a blue cap next to the fuel filler or under the hood. Never put it in the fuel tank, or diesel in the DEF tank: both mistakes can cause very expensive damage. If you let it run low, the vehicle gives plenty of warning, then, by law, limits your speed or won’t restart until you top it up. North American heavy trucks have used DEF since 2010, and most diesel pickups since the early 2010s.