I was just wondering how a diesel engine starts, given that there are no spark plugs. How does it build up 'momentum' for that first explosion in the cylinder? Does it have something to do with the starter motor?
Diesels are started by using a combination of "glow plugs" and a starter motor. Once the diesel gets going, combustion is caused by compression of the mixture. Initially, the cylinder walls can be so cold that they cool the mixture during compression enough to not have it ignite. Each cylinder has a glow plug that provides a small spot of hot surface that is sufficient to ignite the compressed mixture, even when the cylinder walls have cooled it.
Diesel combustion does not involve a mixture. Fuel/air mixtures apply to the Otto cycle in gasoline engines. In diesels, air is compressed to top-dead-center where temperatures can reach 1000 degrees Fahrenheit, and then the fuel is injected and subsequently combusts, resulting in a power stroke.
1) Glow plugs. These have already been covered in another answer. It is a small, resistive plug that current flows through and it heats up. Older ones are just dumb ceramic heaters that control current with V=IR. The newer ones are PWM controlled to use less energy and heat the cylinder air to the right temp. Example
2) Intake heaters. Some larger engines don't have space in the head for glow plugs or they wouldn't work reliably in the environment. These engines have intake air heaters. They heat the air prior to it going into the cylinder. Example
3) Ether. Ether has a much lower autoignition temperature. On very large diesel engines (such as those used in mining trucks) the factory or 3rd party ECM will dose an appropriate amount of ether to assist in getting the engine started. Example
Glow plugs are used on small diesels, like those in cars. For larger diesel engines, like those in locomotives, the engines use either compressed air or hydraulic starters to spin the engine over until it starts running. You can see some of these engines being started on You Tube.
If the engine does not start the first time, usually there is a means for someone to use a hand pump to recharge either the air bottle or the hydraulic fluid reservoir to get another start. These are for emergencies, typically.