What makes the bike so good for special forces is the hybrid electric engine. For normal use, traveling far from sensitive areas, the bike can use diesel, gasoline, or jet fuel, scavenging fuel as need be on missions far from base. When needed, the electric part of the engine ensures that the bike runs silent—or nearly so. DARPA's goal is for the silent mode to ring in at 55 decibels. That's quieter than a normal conversation. The bike's louder mode could reach up to 75 decibels, which is slightly louder than the average vacuum cleaner.