Some engines have glow plugs because their compression ratio is to low to start the engine without them. Your Mercedes Benz 617 will typically have a compression ratio of 26:1 when new and this falls over time. My Dodge Ram high output Cummins has a compression ratio of 30:1. Diesel engines squeeze the air making it real hot and larger engines and those with compression ratios of 35 or 40:1 often do not need glow plugs! Also, your Mercedes Benz will be started and stopped several times in a typical day and industrial diesels will run at a steady speed for days at a time. When the air/fuel mixture is squeezed really hard, it will burst into flame. It really isn't good for high compression diesels to be used for stop and go driving. Diesel engines used to drive freezer compressors often have a compression ratio of 50:1 or even more!! These engines do not need glow plugs or intake heaters because once they are started, they won't be shut down until it's time for their maintenance schedule. They go 24/7 nonstop until service time and at a constant RPM! Does this help?