Chuck Husick: Techno-Talk, March 2004, BoatUS Magazine - Updated January 2009

Diesel Engines
The new federal diesel engine emissions rules are now in full effect for all new diesel engines and any new production engine you buy, either as a stand-alone unit or in a new boat will be in compliance with the current EPA regulations. Manufacturers are moving ahead with production and the diesel engine in the next boat you buy may well meet the 2006 specifications. The new rules cover marine diesels producing more than 37 kW (50 hp) and will be phased in from 2006–2009. Existing engines will be “grandfathered” and can continue in service without alteration.

The new engine emission rules impose lower limits on the amount of hydrocarbon (HC), nitrous oxides (NOx) and particulate matter (PM) permitted in the exhaust, leaving the already low carbon monoxide and carbon dioxide requirements virtually unchanged. The greatest challenge imposed by the new rules is a 22% reduction in combined HC+NOx. (More stringent limits are specified for highway diesels due to their far more substantial numbers and vastly greater operating hours.)

Our experience with these new engines is very different from the highly negative results we remember from initial efforts to clean up our car engines — hard starting, poor acceleration, generally poor fuel economy and unreliability. The new diesels will start more easily, idle much more quietly and smoothly, deliver superior acceleration and provide a small, but welcome, increase in fuel efficiency.

The new technology engines will be totally computer controlled and therefore benefit from the extensive performance monitoring and diagnostic capability of the powerful microprocessors and sophisticated software that manages every aspect of their operation. Virtually any required troubleshooting is accomplished via computer, and is far more precise than is possible with mechanically controlled engines, saving the owner money to hopefully offset the increased cost of the engine.

Meeting the new specifications required a number of technical changes in engine design, primarily in how diesel fuel is burned in the cylinder. A diesel engine’s fuel is injected into the cylinder at high pressure beginning just before the piston reaches its most upward position during the compression stroke. In today’s engines, the injection pressure is usually about 3500 psi and the flow of fuel is continuous once the fuel pressure is high enough to force the fuel injector’s valve open.

In the new engines the injection pressure can be 1600 bar (1600 times atmospheric pressure – 23,500 psi) creating a fuel aerosol much finer than can be attained at today’s much lower injection pressures. The new fuel injectors are electrically controlled by the engine’s computer and deliver fuel in multiple bursts, each lasting perhaps 1/1000th of a second.

The quiet, almost gas engine-like idle sound on the new diesels is a result of beginning the combustion cycle by injecting a small amount of fuel to initiate combustion in the cylinder without causing the very fast pressure rise, which creates the characteristic diesel “rap” sound. The initial puff of fuel is followed by additional injections of however much fuel is needed to meet the load imposed on the engine.

When you inspect one of the new engines you may notice the absence of the familiar fuel injection pump and the individual high-pressure fuel lines that connect it to each fuel injector. In its place, usually under a protective metal cover, you will find a single highly pressurized fuel reservoir, the “common rail” with fuel supply pipes connected to each electrically controlled fuel injector. Some engines dispense with the common rail, using individual unit injectors that operate at fuel pressures equal to that used by the common rail engines. This design approach is most common in large engines where the benefit of eliminating long, high-pressure fuel lines is of greatest value.

The engine control computer manages fuel injection to a precision not attainable with the mechanical controls used on current production engines. The engine can achieve maximum rate acceleration under load without spewing clouds of black smoke (unburned fuel) from the exhaust. The characteristic soot stained transom of some diesel powered boats will be come an increasingly rare sight. The computer will protect the engine from overloading, extending its life. The introduction of ultra-low sulfur diesel fuel (ULSD) in the US will further improve the "livability" of the diesel engine by reducing the impact of engine exhaust on the environment. However, the refining process needed to produce ULSD does make the fuel more expensive by as much as $1.00 a gallon according to some US Government estimates.

The introduction of these new engines presents us with a rare coincidence: enlightened environmental rule making coinciding with advanced technology to produce a clearly superior product.

By Chuck Husick

Chuck Husick is a pilot, engineer, sailor and former president of Chris Craft Boats.

© Copyright BoatUS Magazine 2004 (updated January 2009)