Hydrolock is an abnormal condition of any device which is designed to compress a gas by mechanically restraining it; most commonly the reciprocating internal combustion engine, the case this article refers to unless otherwise noted. Hydrolock occurs when a volume of liquid greater than the volume of the cylinder at its minimum (end of the piston's stroke) enters the cylinder. Since most common liquids are uncompressable the piston cannot complete its travel; either the engine must stop rotating or a mechanical failure must occur.

Symptoms and damage

If an engine hydrolocks while at speed, a mechanical failure is likely. Common damage modes include bent or broken connecting rods, a fractured head, a fractured block, crankcase damage, damaged bearings, or any combination of these. Forces absorbed by other interconnected components may cause additional damage. Physical damage to metal parts can manifest as a "crashing" or "screeching" sound and usually requires replacement of the engine or a substantial rebuild of its major components.

If an internal combustion engine hydrolocks while idling or under low power conditions, the engine may stop suddenly with no immediate damage. In this case the engine can often be purged by unscrewing the spark plugs or injectors and spinning the engine to expel the liquid from the combustion chambers and then restarted. Depending on how the liquid was introduced to the engine, it possibly can be restarted and dried out with normal combustion heat, or it may require more work, such as flushing out corrupted operating fluids and replacing damaged gaskets.

If a cylinder fills with liquid while the engine is turned off, the engine will refuse to turn when a starting cycle is attempted. Since the starter mechanism's torque is normally much lower than the engine's operating torque and momentum this will usually not damage the engine but may burn out the starter. The engine can be drained as above and restarted. If a corrosive substance such as water has been in the engine long enough to cause rusting, more extensive repairs will be required.

Amounts of water significant enough to cause hydrolock tend to upset the air/fuel mixture in gasoline engines. If water is introduced slow enough this effect can cut power and speed in an engine to a point that when hydrolock actually occurs it does not cause catastrophic engine damage.

Automotive

Hydrolock most commonly occurs in automobiles when driving through floods, either where the water is above the level of the air intake or the vehicle's speed is excessive, creating a tall bow wave

Bow wave

A bow wave is the wave that forms at the bow of a ship when it moves through the water. As the bow wave spreads out, it defines the outer limits of a ship's wake. A large bow wave slows the ship down, poses a risk to smaller boats, and in a harbor can cause damage to shore facilities and moored ships...

. A vehicle fitted with a cold air intake

Cold air intake

A cold air intake is an aftermarket assembly of parts used to bring relatively cool air into a car's internal-combustion engine.Most vehicles manufactured since the mid-1970s have thermostatic air intake systems that regulate the temperature of the air entering the engine's intake tract, providing...

 mounted low on the vehicle will be especially vulnerable to hydrolocking when being driven through standing water or heavy precipitation. Engine coolant entering the cylinders through various means (such as a blown head gasket

Head gasket

A head gasket is a gasket that sits between the engine block and cylinder head in an internal combustion engine. Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in...

) is another common cause. Excessive fuel entering (flooding

Flooded engine

A flooded engine is an internal combustion engine that has been fed an excessively rich air-fuel mixture that cannot be ignited. This is caused by the mixture exceeding the upper explosive limit for the particular fuel. An engine in this condition will not start until the excessively rich mixture...

) one or more cylinders in liquid form due to or other abnormal operating conditions can also cause hydrolock.

Marine

Small boats with outboard engines and PWC

Personal water craft

A personal water craft , also called water scooter, is a recreational watercraft that the rider rides or stands on, rather than inside of, as in a boat....

s tend to ingest water simply because they run in and around it. During a rollover or when a wave washes over the craft its engine can hydrolock, though severe damage is rare due to the special air intakes and low rotating inertia of small marine engines. Inboard marine engines have a different vulnerability as these often have their cooling water mixed with the exhaust gases in the header to quiet the engine. Rusted out exhaust headers or lengthy periods of turning the starter can cause water to build up in the exhaust line to the point it back-flows through the exhaust manifold and fills the cylinders.

Diesel engines

Diesel engines are more susceptible to hydrolock than gasoline engines. Diesel engines have a much smaller final combustion chamber volume, requiring much less liquid to hydrolock. Diesel engines also tend to have higher torque

Torque

Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....

, rotating inertia, and stronger starter motors than gasoline engines. The result is that a diesel engine is more likely to suffer catastrophic damage. Diesels can also hydrolock if water builds up in the injector systems and is injected into a running engine in place of fuel.

Radial and Inverted engines

Hydrolock is common on radial

Radial engine

The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel...

 and inverted engines (cylinders pointing downwards) when the engine sits for a long period. Engine oil seeps down under gravity into the cylinder through various means (through the rings, valve guides, etc) and can fill a cylinder with enough oil to hydrolock it. The seepage effect can be observed by the blue-white smoke commonly seen when a radial engine starts up. In order to prevent engine damage, it is universal practice for the ground crew or pilot to check for hydrolock during pre-flight inspection of the aircraft, typically by hand-cranking the propeller for several turns to make sure the crankshaft cycles normally through all cylinders.

Steam engines

A hydraulic lock can occur in steam engine

Steam engine

A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...

s due to steam condensing back into water. In most steam engine designs there must be a small moment at the end of the return stroke of the piston when all the valves are shut and it is compressing any remaining steam. Water can be introduced from the boiler or in a cold engine, steam will condense to water on the cool walls of the cylinders and can potentially hydrolock an engine. This is just as damaging as it is to internal combustion engines and in the case of a steam locomotive can be very dangerous as a broken connecting rod could puncture the firebox/boiler and cause a steam explosion. Steam engines (with the exception of small model and toy machines) are always fitted with cylinder drain cocks which are opened to allow excess water and steam to escape during warm up.