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Schnuerle porting [1] [2] is a system to improve efficiency of a valveless two-stroke engine by giving better scavenging. The intake and exhaust ports cut in the cylinder wall are shaped to give a more efficient transfer of intake and exhaust gases.
Gas flow within the two-stroke engine is even more critical than for a four-stroke engine, as the exhaust flow exits the chamber as the intake enters simultaneously. A well-defined flow pattern is required, avoiding any turbulent mixing. The efficiency of the two-stroke engine depends on effective scavenging, the more complete replacement of the old spent charge with a fresh charge.
Apart from large diesels with separate superchargers, two-stroke engines are generally piston-ported and use their crankcase beneath the piston for compression. The cylinder has a transfer port (inlet from crankcase to cylinder) and an exhaust port cut into it. These are opened, as the piston moves downwards past them; with the higher exhaust port opening earlier as the piston descends; and closing later as the piston rises.
The simplest arrangement is a single transfer and single exhaust port, opposite each other. This "cross scavenging" performs poorly, as there is tendency for the flow to pass from the inlet directly to the exhaust, wasting some of the fuel mixture and also poorly scavenging the upper part of the chamber. Before Schnuerle porting, a deflector on top of the piston was used to direct the gas flow from the transfer port upwards, in a U-shaped loop around the combustion chamber roof and then down and out through the exhaust port. Apart from the gas flow never quite following this ideal path and tending to mix instead, this also gave a poorly shaped combustion chamber with long, thin flame paths.
In 1926, the German engineer Adolf Schnürle developed the system of ports that bears his name. The ports were relocated to both be on the same side of the cylinder, with the transfer port being split into two angled ports, one on either side of the exhaust port. A deflector piston was no longer required. The gas flow was now a circular loop, flowing in and across the piston crown from the transfer ports, up and around the combustion chamber and then out through the exhaust port. [1]
With Schnuerle porting, the piston crown may be of any shape, even bowl shaped. This permits a far better combustion chamber shape and flame path, giving better combustion, particularly at high speeds.
As Schnuerle porting encourages flow in a loop, it is termed "loop scavenging". [3]
Historically, the deflector piston form of cross scavenging was termed "loop scavenging", after the supposed shape of the flow. Schnuerle flow was termed "reverse loop scavenging". [1] As the first of these was realised to be inaccurate, the later form adopted the simpler name. These original terms are now obsolete and no longer used. [1]
The system is named after its inventor, Adolf Schnürle . Either "Schnürle" or the more common Anglicisation as "Schnuerle" are generally acceptable. It also appears as "Schnürrle", [1] but "Schneurle" is a misspelling.
Adolf Schnürle was a prolific engineer and is named on many patent documents. [4]
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In a pump, the function is reversed and force is transferred from the crankshaft to the piston for the purpose of compressing or ejecting the fluid in the cylinder. In some engines, the piston also acts as a valve by covering and uncovering ports in the cylinder.
A two-strokeengine is a type of internal combustion engine that completes a power cycle with two strokes of the piston during one power cycle, this power cycle being completed in one revolution of the crankshaft. A four-stroke engine requires four strokes of the piston to complete a power cycle during 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.
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.
An opposed-piston engine is a piston engine in which each cylinder has a piston at both ends, and no cylinder head. Petrol and diesel opposed-piston engines have been used mostly in large-scale applications such as ships, military tanks, and factories. Current manufacturers of opposed-piston engines include Cummins, Achates Power and Fairbanks-Morse Defense (FMDefense).
In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.
An inlet manifold or intake manifold is the part of an internal combustion engine that supplies the fuel/air mixture to the cylinders. The word manifold comes from the Old English word manigfeald and refers to the multiplying of one (pipe) into many.
Reed valves are a type of check valve which restrict the flow of fluids to a single direction, opening and closing under changing pressure on each face. Modern versions often consist of flexible metal or composite materials.
On a two-stroke engine, an expansion chamber or tuned pipe is a tuned exhaust system used to enhance its power output by improving its volumetric efficiency.
In a piston engine, the crankcase is the housing that surrounds the crankshaft. In most modern engines, the crankcase is integrated into the engine block.
The two-stroke power valve system is an improvement to a conventional two-stroke engine that gives a high power output over a wider RPM range.
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.
A crankcase ventilation system (CVS) removes unwanted gases from the crankcase of an internal combustion engine. The system usually consists of a tube, a one-way valve and a vacuum source.
Cylinder head porting refers to the process of modifying the intake and exhaust ports of an internal combustion engine to improve their air flow. Cylinder heads, as manufactured, are usually suboptimal for racing applications due to being designed for maximum durability. Ports can be modified for maximum power, minimum fuel consumption, or a combination of the two, and the power delivery characteristics can be changed to suit a particular application.
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.
The split-cycle engine is a type of internal combustion engine.
Scavenging is the process of replacing the exhaust gas in a cylinder of an internal combustion engine with the fresh air/fuel mixture for the next cycle. If scavenging is incomplete, the remaining exhaust gases can cause improper combustion for the next cycle, leading to reduced power output.
In internal combustion engines, a split-single design is a type of two-stroke where two cylinders share a single combustion chamber.
Internal combustion engines come in a wide variety of types, but have certain family resemblances, and thus share many common types of components.
In an internal combustion engine, the geometry of the exhaust system can be optimised ("tuned") to maximise the power output of the engine. Tuned exhausts are designed so that reflected pressure waves arrive at the exhaust port at a particular time in the combustion cycle.
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.