The Average Lifespan Of A Diesel Engine Is Longer Than You Might Realize

Whether you call them semi trucks, big rigs, or 18-wheelers, the large commercial trucks that crisscross the nation's highways with important freight are built to last. In fact, it's not uncommon for their large diesel engines to travel more than one million miles between rebuilds with proper care. This begs the question, how long can we expect the diesel powerplants in lighter grade — but still heavy duty — consumer pickup trucks to last?

According to Capital One's Auto Navigator website, diesel engines typically enjoy an average lifespan that's approximately double that of their gasoline counterparts. While caution is perhaps warranted as these vehicles approach a half-million miles, a few hundred thousand seems like a perfectly attainable goal. So what's the key to diesel's longevity?

A typical gasoline engine takes a mixture of fuel and air, compresses it, and then ignites the compressed mixture with a spark, causing an explosion, which is called combustion. The diesel engine — invented by German engineer Rudolf Diesel in the 1890s — takes a different approach. A diesel compresses air and then injects fuel into the compressed air. No spark (hence, no spark plug) is necessary because the heat of the compressed air ignites the fuel spontaneously.

A key component of significantly compressing air in a diesel engine such that ignition doesn't require a spark is a very high compression ratio — typically about twice that of a gasoline engine. On the surface, a higher compression ratio might seem like a source of additional stress to an engine's internals, but components like the crankshaft and bearings are built to be incredibly robust to withstand the abuse. 

Besides stout construction, diesel engines also operate at lower RPMs, which prolongs longevity, as does a higher oil capacity and advanced oiling techniques, such as oil jets to spray the undersides of pistons on many engines. Finally, diesel fuel itself is not as refined as gasoline and therefore has more lubricity, which leads to slower wear on an engine's cylinder bores.

According to diesel parts retailer prosourcediesel, Chevrolet and GMC pickup trucks with Duramax diesel powerplants need to reach approximately 300,000 miles before being considered "high mileage," but 500,000 miles is not out of the question with meticulous maintenance. Reviews of Ford's Powerstroke and RAM's Cummins diesel engines yield near-identical results, with an average lifespan in the 300,000 to 350,000-mile range. 

If there's a downside to these long-lasting motors, it's that the rest of your truck such as the body and suspension may not be able to withstand the many years and miles as well as the engine, but in retrospect, that's a great problem to have.