Push starting, also known as bump starting, roll starting, clutch starting, popping the clutch or crash starting, is a method of starting a

motor vehicle A motor vehicle, also known as a motorized vehicle, automotive vehicle, automobile, or road vehicle, is a self-propelled land vehicle, commonly wheeled, that does not operate on railway track, rails (such as trains or trams), does not fly (such ...

with an

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

that has a

manual transmission A manual transmission (MT), also known as manual gearbox, standard transmission (in Canadian English, Canada, British English, the United Kingdom and American English, the United States), or stick shift (in the United States), is a multi-speed ...

, a mechanical fuel pump, and a mechanically driven generator or alternator. By pushing or letting the vehicle roll downhill then engaging the clutch at the appropriate speed the engine will turn over and start. The technique is most commonly employed when other starting methods (

automobile self starter A starter (also self-starter, cranking motor, or starter motor) is a device used to rotate (crank) an internal-combustion engine so as to initiate the engine's operation under its own power. Starters can be electric, pneumatic, or hydraulic. T ...

kick start Kick start is a method of starting an internal combustion engine (usually that of a motorcycle) by pushing a ratcheting lever with the rider's foot. Kick start mechanisms were almost universally a part of motorcycle engines before the mid-1970s ...

, jump start etc.) are unavailable. The most common way to push start a vehicle is to put the

in second gear, switching the ignition to on/run, depressing the clutch, and pushing the vehicle until it is at a speed of or more, then quickly engaging the clutch to make the engine rotate and start while keeping the gas pedal partially depressed, then quickly disengaging the clutch so it does not stall.

Types

Push starting is most successful when the automobile is using a

gasoline Gasoline ( North American English) or petrol ( Commonwealth English) is a petrochemical product characterized as a transparent, yellowish, and flammable liquid normally used as a fuel for spark-ignited internal combustion engines. When for ...

engine, uses a

carburetor A carburetor (also spelled carburettor or carburetter) is a device used by a gasoline internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the Ventu ...

, and uses a

capacitor discharge ignition Capacitor discharge ignition (CDI) or thyristor ignition is a type of automotive electronic ignition system which is widely used in outboard motors, motorcycles, lawn mowers, chainsaws, small engines, gas turbine-powered aircraft, and some cars ...

(CDI) or an inductive discharge ignition system. Automobiles with other types of engine, ignition, and fuel delivery configurations may work, but may be more difficult to start. Some engines must have a battery providing some electricity since

fuel injection Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All c ...

systems must have power to operate.

Automatic or manual gearbox

A vehicle equipped with an

automatic transmission An automatic transmission (AT) or automatic gearbox is a multi-speed transmission (mechanics), transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. The 1904 ...

(including semi-automatic (''clutchless manual'') transmissions) is difficult to push start since selection of transmission gears is possible only when the internals of such a gearbox are rotating. However, automatics with both front and rear hydraulic pumps can be push-started with no problems. The last American automobile with this type of transmission was the 1969 Chevrolet Corvair with a '' Powerglide'' automatic. While push-start can cause more damage to a hydrolocked engine, the starter motor is limited.

Petrol or diesel engines

A diesel engine uses heat and high compression ( compression ratios commonly 16-23:1 versus 8-12:1 for gasoline) to ignite the fuel. When normally starting a modern diesel engine, it typically uses glowplugs to preheat the cylinder(s). If a battery is completely discharged then it may not provide the necessary power to heat the glowplugs, making the push starting of a diesel vehicle with a depleted battery almost impossible.

Fuel delivery systems

Fuel injection is most common for modern gasoline and diesel engines.

Fuel injection Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of a fuel injector. This article focuses on fuel injection in reciprocating piston and Wankel rotary engines. All c ...

needs electrical power to open and close the fuel injectors. If a battery is of a sufficiently discharged state that it cannot provide the power to turn an

then it may also not be possible to activate the injectors. The most common method to start such a vehicle engine is to jump start it. A

fuel pump A Fuel pump is a component used in many liquid-fuelled engines (such as petrol/gasoline or diesel engines) to transfer the fuel from the fuel tank to the device where it is mixed with the intake air (such as the carburetor or fuel inject ...

is used for fuel injection. It can be mechanically driven or electrically driven. If electrical then the same problem may arise which the battery cannot turn the pump because it is heavily discharged. A carburetor only needs suction from the internal combustion engine to work best when push starting. Once the engine is running, a fuel pump (mechanical or electrical) will continue to supply fuel to the carburetor. Carburetor engines may damage the

catalytic converter A catalytic converter part is an vehicle emissions control, exhaust emission control device which converts toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalysis, catalyzing a redox ...

by fuel when cranked longer without igniting fuel or electrical controlling the carburetor.

Ignition systems

A modern gasoline engine contains an electronic ignition system which precisely times the electrical pulse to the spark plug. The advantage of such a device is that it can deliver a full power electrical pulse to the spark plugs even when the

alternator An alternator (or synchronous generator) is an electrical generator that converts mechanical energy to electrical energy in the form of alternating current. For reasons of cost and simplicity, most alternators use a rotating magnetic field wit ...

is turning very slowly (as in push starting a motor). The outdated method of a mechanically timed ignition system is that it cannot deliver a full electrical pulse at very low engine revolutions per minute (

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or r⋅min−1) is a unit of rotational speed (or rotational frequency) for rotating machines. One revolution per minute is equivalent to hertz. Standards ISO 80000-3:2019 def ...

). This may affect the ease of push starting an engine to life.

History

In the early 20th century, many motorcycles could only be push started; the 1908 Scott was distinguished by introducing a

er feature. Excelsior Motor Company's

Welbike The Welbike is a British single-seat folding motorcycle produced during World War II at the direction of Station IX — the "Inter Services Research Bureau" — based at Welwyn, UK, for use by Special Operations Executive (SOE). It has the di ...

, intended to be carried by paratroopers in World War II, was designed to be started only by push starting.

References

{{Reflist Motorcycle engines Starting systems