Vehicle running gear

The Myth of Compression Braking


Recently a remark by an acquaintance triggered me to dredge up a bit of knowledge I came by ages ago, during a short stint driving a diesel tractor/trailer rig for J.C. Penney between Phoenix and Tucson. The fellow, a UK resident, had recently sold his mid-90s Land Rover Discovery V8 with a five-speed manual transmission, and purchased a similar Discovery equipped with a 200 Tdi turbodiesel engine, also with a five-speed. Besides instantly doubling his fuel economy and then some, he’d noticed something else and mentioned it in an email. “I might be daft,” he wrote, “but I swear the petrol engine had better compression braking. But I know that’s impossible. It only had 9:1 compression, and the Tdi is 19:1. Something else must be at work.” 

I wrote him back a short reply: “That’s because diesel engines have no compression braking.” Which caused him to respond, “Now I think you’re daft.”