The heart of the HDT diesel motorcycle is this single-cylinder, four-stroke, indirect-injection, and liquid-cooled 611cc engine. It's rated at 30 hp at 5,700 rpm and 33 lb-ft of torque at 4,200 rpm. Originally designed to run on diesel fuel, biodiesel, or kerosene, the engine relies on the simplicity of mechanical injection.
British engineer Dr. Stuart McGuigan took the lead in developing the new diesel motorcycle engine, one uniquely suited for its mission. The result is a lightweight, liquid-cooled 611cc four-valve DOHC unit, developing 30 hp at 5,700 rpm. While this may not sound like a rousing output, it is more than double that of an equivalent-size industrial diesel engine, with approximately double the rpm range. Hayes said that in testing, the engine freely revved up to an astounding 8,000 rpm and is easily capable of more than 7,000 rpm, shattering the myth that diesel engines are incapable of speeds above 5,500 rpm. And the fuel efficiency is simply unmatched by gasoline-powered equivalents. Bikes equipped with this engine have achieved 96 mpg in military-certified testing, and in a street tune should be capable of 102 mpg, according to Hayes. He said the engine responds well to a variety of heavy fuels, including kerosene, JP8 (the standard fuel of the U.S. military), and biodiesel. Although the engine runs a little smoky on JP8, Hayes says on EPA pump diesel, the smoke is nearly non-existent. Not resting on his laurels, Hayes is already anticipating and preparing for the '08 diesel motorcycle emissions standards and is confident the bike will meet or exceed them.
The engine's penchant for revs belies its compression-combustion design. Other than a somewhat noisy idle, at speed, the engine sounds similar to any other large-displacement single. And although it's no match for a Suzuki Hayabusa or other crotch rockets, it's capable of 90-plus mph, making it more than freeway capable.
You may be thinking, "Why don't they throw a turbo on that thing?" a question that Hayes himself asked. However, for its intended mission, a naturally-aspirated engine was the best fit. "In terms of throttle response, simplicity, and ease of maintenance, we determined a naturally aspirated engine was the way to go. We've tested some turbocharged prototypes, but the complexity, packaging, and heat-dissipation issues that come with a turbo ruled that out for the military bikes. However, down the road, we're looking at a possible V-twin diesel with a turbo option, but that's several years away," Hayes also said that the unique design of the HDT engine gives it a dynamic operating range far higher than most conventional diesels, allowing it to produce peak power where most conventional, even turbocharged diesels fear to tread.
For the civilian motorcycle market, HDT will offer this Bulldog version with a patriotic graphics package. Rated for more than 102 mpg, the Bulldog should be a winner for sport touring, on- and off-road racing, and everyday commuting. Production is scheduled for March of 2006.
Hayes has not yet announced pricing for the civilian model, but said that consumer interest and inquiries have been "overwhelming" so far. It's expected that the price tag of the consumer-ready version will likely carry a 20-to-30-percent premium over an equivalent gasoline-powered motorcycle. Hayes expects to start offering street-legal versions of the bike around April of 2006.