Diesels are also available in both two- and four-stroke versions. Engine cooling Liquid Liquid-cooled motorcycles have a radiator, similar to the radiator on a car, which is the primary way their heat is dispersed. Coolant is constantly circulated between this radiator and the cylinders when the engine is running. While most off-road motorcycles have no radiator fan and rely on air flowing over the radiators from the forward motion of the motorcycle, many road motorcycles have a small fan attached to the radiator which is controlled by a thermostat. Some off-road motorcycles are liquid-cooled, and anti-dirt protection is attached to the radiator. The cooling effect of this fan is enough to prevent the engine overheating in most conditions, so liquid-cooled bikes are safe to use in a city, where traffic may frequently be at a standstill. Emissions regulations and the market demand for maximum power are driving the motorcycle industry to liquid-cooling for most motorcycles. Even Harley-Davidson, a strong advocate of air-cooled motors, has recently added water cooling to the heads of their V-twin motors in their large touring models, with the radiators being hidden in the lower leg fairings. The Harley-Davidson Street 500, Street 750, and two V-Rod models have 60° V-twins that are fully water cooled, as opposed to the traditional air-cooled 45° V-twins. Air Most air-cooled motorcycles take advantage of air blowing past the cylinder and cylinder head while in motion to disperse heat. Frequent, sustained stationary periods may cause overheating. Some models, mostly scooters, are equipped with fans that force the air to go past the cylinder block, which solves the problem of city driving. The cylinders on air-cooled bikes are designed with fin heat sinks to aid in this process. Air-cooled bikes are cheaper, simpler and lighter than their water-cooled counterparts. Oil Some manufacturers use a hybrid cooling method where engine oil is circulated between the engine case and a small radiator. Here the oil doubles as cooling liquid, prompting the name "oil-cooling." Suzuki has produced many oil-cooled motorcycles. Modern BMW R-series flat-twin motorcycles, such as the R1150GS, use air and oil cooling. Polaris's Victory motorcycles use oil/air cooling exclusively. Other components Fuel Injection Fuel injection and computer engine management systems are now normal on middle range and larger motorcycles and are increasingly being incorporated onto the smaller machines, partly driven by better emission control and lower maintenance but mostly by manufacturing cost considerations. Ignition