Diesels do take a lot of power to crank over. While a typical V8 gas engine may use about 300 to 400 amps electrical power in the starter to crank the engine at temperatures below minus 20C, a diesel starter will use almost double that. Most of the power is used to compress the air. Gasoline engines have compression ratios around 9.5:1 (the amount of times the air is squeezed or compressed in the cylinder during the compression stroke). Diesel engines use compression ratios of 16:1 up to about 20:1. The higher compression ratios used in diesel engines gives them better efficiency, but makes them harder to crank.
Diesel vehicles have bigger batteries, larger diameter battery cables and heavy-duty starters for cold weather cranking. Many large V8 diesel-powered vehicles will use two batteries to provide the electrical power for cranking the engine. Just as with gas-powered vehicles, cold weather decreases the battery power available. In colder parts of Canada, battery blankets are often installed around batteries so they can be plugged in at the same time as the block heater to keep the battery warm.
High diesel engine compression ratios do something else - it heats the air, which is used to ignite the fuel. Gasoline engines use sparkplugs to ignite the fuel already in the cylinder. Diesel engines heat the air by compressing it and then fuel is injected into the hot air, which starts it burning immediately. The engine must crank over fast enough to heat the air or a diesel won’t start. Minimum cranking speed for many diesel engines is about 100 rpm. Below that, the air cools off to fast and the fuel doesn’t ignite.
Preheaters are used on most diesel engines to help heat the air during start up. Intake manifold heaters use an electrical coil inside the intake manifold to heat the air as it enters the engine. Glow plugs are individual heaters screwed into each cylinder that glow red hot during engine cranking. Both these heaters operate for only a few seconds during engine starting. If they were to operate for longer periods, they would burn out, which was common before computer controls took over the task of monitoring and operating heater control. If an intake heater or glow plugs are not working, the engine may be difficult to start, run rough and create white smoke during starting and initial operation. Once the cylinders warm up, the engine will run fine.