titanium armor on combat vehicles is increasing. The use of titanium can lighten the vehicle's weight. Appliqué armor can be quickly applied to vehicles and has been utilized on a number of combat vehicles, such as the M8 Armored Gun System. Fire suppression Contemporary combat vehicles may incorporate a fire-suppression system to mitigate damage from fire. Systems can be employed in the engine and crew compartments and portable systems may be mounted inside and outside the vehicle as well. Automatic fire suppression systems activate instantaneously upon the detection of fire and have been shown to significantly improve crew survivability. Halon fire suppression systems quickly inundate an affected fire breach with a flood of halon to extinguish leaking fuel. Halon remains necessary for crew compartment fire suppression due to space and weight constraints, and toxicity concerns. Nitrogen systems take up about twice as much space as a comparable halon unit. Germany uses this system as a replacement for its halon system. Some systems, such as Germany's previous extinguisher, have a second shot of suppressant to mitigate re-ignition or the effects of a second hit. Though not as instantaneous, portable crew-operable extinguishers are also used inside and outside the vehicle. Typically, portable extinguishers use a CO2 agent instead of the halon agents used in the past. CO2 can become lethal to vehicle occupants if it accumulates into a deadly concentration. The U.S. Army has adopted a replacement formula consisting of 50% water, 50% potassium acetate. Alternatives such as powder formulas also exist. Crew and occupants Combat vehicles require at least one crew member, though typically at least two (one driver and one gunner). Some vehicles, such as armored personnel carriers, also contain a dedicated passenger bay, able to carry up to a dozen or more passengers. Hygiene upkeep is difficult when operating a combat vehicle. Mobility Tracked combat vehicles are suited for heavy combat and rough terrain. Wheeled combat vehicles offer improved logistical mobility and optimized speeds on smooth terrain. Silent watch is becoming an increasingly important combat vehicle application. It is a role that requires that all mission requirements be met while keeping acoustic and infrared signature levels to a minimum. For this reason, silent watch often requires the vehicle to operate without use of the main engine and sometimes even auxiliary engines. Many modern combat vehicles often have electronic equipment that cannot be supported solely with auxiliary batteries alone. Auxiliary