City driving increases truck & bus emissions.

Diesel exhaust particulate matter (DPM) is known to cause cancer, and while much of the blame has been placed on ageing truck and bus fleets, Brisbane-based firm Cost Effective Maintenance has concluded that the single biggest contributor to diesel smoke is the compounding effect of city driving cycles.

Short answer: the more city driving a truck or bus does, the more it smokes. Stop-start operation and heavy idling stop a diesel from running at the steady load and temperature it’s designed for, so deposits build up fast — and emissions climb — even on a well-maintained engine.

Why City Driving Causes More Smoke

General manager Brid Walker said: “Put simply, the more city driving a truck or bus does, the more smoke it puts out. Prolonged stop-start operation, with excessive idling in traffic, is typical of congested city driving. Diesel engines are designed to run at a decent load and steady temperature for the best fuel efficiency and lowest emissions. That just doesn’t happen with city driving, and it increases engine deposits. Visible smoke can increase after just a few days of continuous city driving.”

“We’ve repeatedly demonstrated that by removing accumulated crankcase and combustion chamber deposits, and introducing keep-clean technology, DPM can be kept at acceptable levels — even under congested city-type work.”

Most Rebuilds Can Be Avoided

“While a lack of maintenance will undoubtedly make things worse, it’s not directly to blame here. The city duty cycle itself is the single most important factor in diesel particulate emissions,” says Walker. “We’re continually presented with engines that are well serviced and in good, sound condition but are smoking excessively or failing emissions tests. In many cases, the owners have been advised to have their engines rebuilt, at considerable cost, to restore emission control.”

The good news: in reality, most of these rebuilds can be avoided. Cleaning up the deposits and keeping the engine clean usually restores emission control at a fraction of the cost of a rebuild.

This ties in closely with how deposits drive other diesel problems. For more, see our guides on fixing DPF regen problems, engine blowby (most engines don’t need a rebuild), and how FTC Decarbonizer works.