Four-stroking

Last updated

Four-stroking is a condition of two-stroke engines where combustion occurs every four strokes or more, rather than every two. Though normal in some instances at idle, extremely high engine speeds, and when letting off the throttle, such firing is uneven, noisy and may, in cases of malfunction, damage the engine if allowed to continue unabated.

Contents

Four stroking will occur in a correctly adjusted two stroke engine at full throttle without load when the air-fuel mixture becomes overly rich and prevents the engine from running faster. At such high speeds a mixture that is too lean will cause the engine to over-rev as well as overheat, and in engines running on premixed fuel a mixture that is too lean will cause poor lubrication.

In chain saw operation, where natural fluctuation of chain bite during a cut can cause momentary over-revving, the full throttle mixture is adjusted for four-stroking to occur at a set high rpm, cutting engine speed and enriching lubrication.

Causes

Two stroke engines rely on effective scavenging in order to operate correctly. This clears out the combustion exhaust gases from the previous cycle and allows refilling with a clean mix of air and fuel. If scavenging falters, the mixture of unburnable exhaust gas with the new mixture may produce an overall charge that fails to ignite correctly. Only when this charge is enriched by a second volume of clean mixture does it become flammable again. The engine thus begins to fire every second cycle (every four strokes), rather than correctly on every cycle. [1] Four-stroking begins gradually, so the engine first starts to run with an unpredictable mixture of two- and four-stroke cycles. When severe, this may even become six- or eight-stroking. [1]

Scavenging of small two-stroke engines relies on inertial scavenging through the Kadenacy effect. At low rpm and low gasflow velocities, this effect is reduced. Scavenging thus becomes less effective when idling, and so it is when idling (at either low rpm or low throttle) that four-stroking is most likely to become a problem. [1] Schnuerle or loop scavenging is considered to be less prone than the simpler cross-scavenging. [1]

Four-stroking is not caused by an over-rich mixture, as is widely believed, although this can make it worse. [lower-roman 1] [ according to whom? ] Nor is it caused by excessive oil/fuel lubrication mixtures. [lower-roman 2] [ according to whom? ]

Four-stroking in diesels

Four-stroking is less likely with compression ignition engines (i.e. diesel engines) than it is with spark-ignition engines (i.e. petrol engines). Diesel engines are also rare as the small two-strokes where inertial scavenging is used. [lower-roman 3] When large two-stroke diesel engines are used, these have scavenging by forced induction and so are generally immune to four-stroking when idling at low speed or low power. These scavenge blowers may be mechanically-driven Roots blowers or turbochargers. As a turbocharger has some lag time coming up to speed, turbo-charged two-stroke diesels often display four-stroking when starting, or when suddenly accelerating from idle. Some large engines, such as those from EMD, minimize this by using a turbocharger with an auxiliary mechanical drive to give better scavenging at low rpm.

Model engines

Four-stroking is a common and expected behaviour with model engines, both glow fuel and diesel. These small engines rely on scavenging at their extremely high rotational speeds. When started, they run as inertially-scavenged four strokes and have a distinctive change in engine note when they accelerate past the point at which they begin to operate as two strokes. Owing to the scaling laws of such small engines, this four-stroking is an unavoidable consequence of limitations on their scavenging at slow speeds. However the same scaling laws also make the effects of four-stroking less severe and so the engines can idle satisfactorily in this mode without damage.

The pilots of control line aerobatic model aircraft often depend on "four-stroking" of their glow fuel burning two stroke model engines for optimal flight performance, including with fixed-venturi four stroke model engines.

Hazards of four-stroking

When a four-stroking engine eventually fires, the excess mixture from the previous failed combustion stroke causes an excessive cylinder pressure. This can be nearly double the normal pressure, leading to excess noise and potentially failure of overloaded bearings in the connecting rod. [2]

Four-stroking is particularly noisy, especially as it occurs when the engine is otherwise relatively quiet and a vehicle potentially stationary at idle. In some cases, particularly with two-stroke engines installed in cars, extra exhaust silencing may be installed to offset this. [3]

Avoiding four-stroking

Four-stroking is problematic when a quiet, docile engine is required and also when the load on an engine suddenly changes. [4]

Retarding ignition timing reduces four-stroking, as it allows more time for scavenging to take effect. Two-stroke engines, when running at low power, are less sensitive to ignition timing changes than four-stroke engine. If the timing is retarded for low throttle positions, from perhaps 35° before top dead center at normal speeds to top dead center or even 10° after TDC at slow idle, the engine runs well without four-stroking. Opening up the throttle and simultaneously advancing the timing, allows a rapid pickup in speed. This system is widely used for marine outboard motors, particularly when used to pull water skiers. [1]

Twin- or multi-cylinder engines may improve low-speed scavenging to one cylinder by shutting down the other cylinder at low speeds. This may be done simply by cutting the ignition spark to one, thus increasing the load on the other cylinder and thus the power and gas-flow required. [5] This has the drawback of wasting fuel in the un-ignited cylinder, potentially also risking oiling-up its spark plug and clogging the exhaust system. More sophisticatedly, as was done for some Johnson outboard motors, the transfer passage for one cylinder may be closed by an additional throttle butterfly, shutting that cylinder off completely and isolating it from the fuel air mixture. [5] This avoids the risk of oil-fouling and routes all gas-flow through the operating cylinder, greatly increasing fuel economy.

Notes

  1. This is generally caused by a blocked air filter, as this also reduces airflow (and thus scavenging) and the slide carburettors used on motorcycles tend to supply an over-rich fuel mixture if the airflow is restricted.
  2. This is a confusion from two-stroking, when a four-stroke diesel engine with an over-filled oil sump may begin to run out of control by sucking excess oil into the cylinders and burning this as an impromptu fuel.
  3. Where small diesels are used as small engines and for the increasing interest in diesel motorcycles, they are mostly four-strokes.

Related Research Articles

<span class="mw-page-title-main">Miller cycle</span> Thermodynamic cycle

In engineering, the Miller cycle is a thermodynamic cycle used in a type of internal combustion engine. The Miller cycle was patented by Ralph Miller, an American engineer, U.S. patent 2,817,322 dated Dec 24, 1957. The engine may be two- or four-stroke and may be run on diesel fuel, gases, or dual fuel. It uses a supercharger or a turbocharger to offset the performance loss of the Atkinson cycle.

<span class="mw-page-title-main">Two-stroke engine</span> Internal combustion engine type

A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston in one revolution of the crankshaft. A four-stroke engine requires four strokes of the piston to complete a power cycle in two crankshaft revolutions. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust functions occurring at the same time.

<span class="mw-page-title-main">Exhaust gas recirculation</span> NOx reduction technique used in gasoline and diesel engines

In internal combustion engines, exhaust gas recirculation (EGR) is a nitrogen oxide (NOx) emissions reduction technique used in petrol/gasoline, diesel engines and some hydrogen engines. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. The exhaust gas displaces atmospheric air and reduces O2 in the combustion chamber. Reducing the amount of oxygen reduces the amount of fuel that can burn in the cylinder thereby reducing peak in-cylinder temperatures. The actual amount of recirculated exhaust gas varies with the engine operating parameters.

<span class="mw-page-title-main">Four-stroke engine</span> Internal combustion engine type

A four-strokeengine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

  1. Intake: Also known as induction or suction. This stroke of the piston begins at top dead center (T.D.C.) and ends at bottom dead center (B.D.C.). In this stroke the intake valve must be in the open position while the piston pulls an air-fuel mixture into the cylinder by producing a partial vacuum in the cylinder through its downward motion.
  2. Compression: This stroke begins at B.D.C, or just at the end of the suction stroke, and ends at T.D.C. In this stroke the piston compresses the air-fuel mixture in preparation for ignition during the power stroke (below). Both the intake and exhaust valves are closed during this stage.
  3. Combustion: Also known as power or ignition. This is the start of the second revolution of the four stroke cycle. At this point the crankshaft has completed a full 360 degree revolution. While the piston is at T.D.C. the compressed air-fuel mixture is ignited by a spark plug or by heat generated by high compression, forcefully returning the piston to B.D.C. This stroke produces mechanical work from the engine to turn the crankshaft.
  4. Exhaust: Also known as outlet. During the exhaust stroke, the piston, once again, returns from B.D.C. to T.D.C. while the exhaust valve is open. This action expels the spent air-fuel mixture through the exhaust port.
<span class="mw-page-title-main">Aircraft engine controls</span>

Aircraft engine controls provide a means for the pilot to control and monitor the operation of the aircraft's powerplant. This article describes controls used with a basic internal-combustion engine driving a propeller. Some optional or more advanced configurations are described at the end of the article. Jet turbine engines use different operating principles and have their own sets of controls and sensors.

The GM Ecotec engine, also known by its codename L850, is a family of all-aluminium inline-four engines, displacing between 1.4 and 2.5 litres. Confusingly, the Ecotec name was also applied to both the Buick V6 Engine when used in Holden Vehicles, as well as the final DOHC derivatives of the previous GM Family II engine; the architecture was substantially re-engineered for this new Ecotec application produced since 2000. This engine family replaced the GM Family II engine, the GM 122 engine, the Saab H engine, and the Quad 4 engine. It is manufactured in multiple locations, to include Spring Hill Manufacturing, in Spring Hill, Tennessee, with engine blocks and cylinder heads cast at Saginaw Metal Casting Operations in Saginaw, Michigan.

<span class="mw-page-title-main">Engine braking</span> Retarding forces within an engine used to slow a vehicle

Engine braking occurs when the retarding forces within an internal combustion engine are used to slow down a motor vehicle, as opposed to using additional external braking mechanisms such as friction brakes or magnetic brakes.

Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere.

The anti-lag system (ALS) is a method of reducing turbo lag or effective compression used on turbocharged engines to minimize turbo lag on racing or performance cars. It works by delaying the ignition timing and adding extra fuel to balance an inherent loss in combustion efficiency with increased pressure at the charging side of the turbo. This is achieved as an excess amount of fuel/air mixture escapes through the exhaust valves and combusts in the hot exhaust manifold spooling the turbocharger creating higher usable pressure.

<span class="mw-page-title-main">Bourke engine</span> Type of internal combustion engine

The Bourke engine was an attempt by Russell Bourke, in the 1920s, to improve the two-stroke internal combustion engine. Despite finishing his design and building several working engines, the onset of World War II, lack of test results, and the poor health of his wife compounded to prevent his engine from ever coming successfully to market. The main claimed virtues of the design are that it has only two moving parts, is lightweight, has two power pulses per revolution, and does not need oil mixed into the fuel.

<span class="mw-page-title-main">Hot-bulb engine</span> Internal combustion engine

The hot-bulb engine, also known as a semi-diesel, is a type of internal combustion engine in which fuel ignites by coming in contact with a red-hot metal surface inside a bulb, followed by the introduction of air (oxygen) compressed into the hot-bulb chamber by the rising piston. There is some ignition when the fuel is introduced, but it quickly uses up the available oxygen in the bulb. Vigorous ignition takes place only when sufficient oxygen is supplied to the hot-bulb chamber on the compression stroke of the engine.

<span class="mw-page-title-main">Ignition timing</span> Timing of the release of a spark in a combustion engine

In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.

<span class="mw-page-title-main">Model engine</span>

A model engine is a small internal combustion engine typically used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, free flight, control line aircraft, or ground-running tether car model.

<span class="mw-page-title-main">Hit-and-miss engine</span> Obsolete type of gasoline engine

A hit-and-miss engine or Hit 'N' Miss is a type of stationary internal combustion engine that is controlled by a governor to only fire at a set speed. They are usually 4-stroke but 2-stroke versions were made. It was conceived in the late 19th century and produced by various companies from the 1890s through approximately the 1940s. The name comes from the speed control on these engines: they fire ("hit") only when operating at or below a set speed, and cycle without firing ("miss") when they exceed their set speed. This is as compared to the "throttle governed" method of speed control. The sound made when the engine is running without a load is a distinctive "Snort POP whoosh whoosh whoosh whoosh snort POP" as the engine fires and then coasts until the speed decreases and it fires again to maintain its average speed. The snorting is caused by the atmospheric intake valve used on many of these engines.

Diesel engine runaway is an occurrence in diesel engines, in which the engine draws extra fuel from an unintended source and overspeeds at higher and higher RPM, producing up to ten times the engine's rated output until destroyed by mechanical failure or bearing seizure due to a lack of lubrication. Hot-bulb engines and jet engines can also run away via the same process.

<span class="mw-page-title-main">Two-stroke diesel engine</span> Engine type

A two-stroke diesel engine is a diesel engine that uses compression ignition in a two-stroke combustion cycle. It was invented by Hugo Güldner in 1899.

<span class="mw-page-title-main">Carbureted compression ignition model engine</span> Type of carbureted engine

A carbureted compression ignition model engine, popularly known as a model diesel engine, is a simple compression ignition engine made for model propulsion, usually model aircraft but also model boats. These are quite similar to the typical glow-plug engine that runs on a mixture of methanol-based fuels with a hot wire filament to provide ignition. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engines.

Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.

<span class="mw-page-title-main">Internal combustion engine</span> Engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber

An internal combustion engine is a heat engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal combustion engine, the expansion of the high-temperature and high-pressure gases produced by combustion applies direct force to some component of the engine. The force is typically applied to pistons, turbine blades, a rotor, or a nozzle. This force moves the component over a distance, transforming chemical energy into kinetic energy which is used to propel, move or power whatever the engine is attached to.

References

Works cited