Tuned exhaust
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.
Two-stroke engines
Expansion chambers
In two-stroke engines where the exhaust port is opened by being uncovered by the piston (rather than by a separate valve), a tuned exhaust system usually consists of an expansion chamber. The expansion chamber is designed to produce a negative pressure wave to assist in filling the cylinder with the next intake charge, and then to produce a positive pressure wave which reduces the amount of fresh intake charge that escapes through the exhaust port (port blocking).
Uniflow scavenging
An alternate design of two-stroke engines is where the exhaust port is opened/closed using a poppet valve and the intake port is piston-controlled (opened by being uncovered by the piston). The timing of the exhaust valve closure is designed to assist in filling the cylinder with the next intake charge (as per four-stroke engines).
An
Four-stroke engines
In a four-stroke engine, an exhaust manifold which is designed to maximise the power output of an engine is often called "extractors" or "headers". The pipe lengths and merging locations are designed to assist in filling the cylinder with the next intake charge using exhaust scavenging.[1] Locations where exhaust pipes from individual cylinders merge are called "collectors". The diameters of the exhaust system are designed to minimise back-pressure by optimising the gas velocity.
Extractors/headers usually have equal length pipes for each cylinder, whereas a more basic exhaust manifold may have unequal length pipes.
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Exhaust system for a Rotax 912s airplane engine
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Long tube headers (in white) on a racing car
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"Zoomie" headers on a dragster
4-2-1 exhausts
A 4-2-1 exhaust system is a type of
Compared with a 4-1 exhaust system, a 4-2-1 often produces more power at mid-range engine speeds (RPM), while a 4-1 exhaust produces more power at high RPM.[3][4]
Cylinder pairings
The purpose of a 4-2-1 exhaust system is to increase scavenging by merging the exhaust paths of specific pairs of cylinders. Therefore, the cylinder pairings are defined by the intervals between firing events, which is determined by the firing order and— for engines with an unevenly spaced firing order— the firing interval.
For an
For a V8 engine with a typical crossplane design, 4-2-1 exhausts are often called "Tri-Y" exhausts. Traditionally, only cylinders within the same bank were paired, resulting in spacings of 90-630 degrees (sequential), 180-540 degrees or 270-450 degrees. Typically, the 270 interval is favoured, requiring different pairings in each bank; e.g. 1 & 2 and 3 & 4 in one bank, but 1 & 3 and 2 & 4 in the other. Naturally, such exhausts are sensitive to the specific firing order in use.[5][6] Even spacings of 360-360 degrees are only possible if a cross-over exhaust manifold is used to pair cylinders from separate banks.
Pipe lengths
The combining of exhaust pressure pulses from each cylinder dictates the lengths of the pipes necessary. Generally, shorter pipes will help produce more power at higher engine rpm, and longer pipes favour low-rpm torque, thereby altering the power band.[7] However, the gases tend to cool as they pass through longer pipes, which reduces the effectiveness of the catalytic converter.
In a turbocharged engine, the key factor in the length of the exhaust pipes is providing evenly-spaced pressure pulses to the turbine of the turbocharger.[5]
See also
Two-stroke engines |
Four-stroke engines
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References
- Philip H. Smith, pp137-138
- ^ "What does a 4-2-1 exhaust system do?". www.howstuffworks.com. 21 December 2011. Retrieved 24 September 2019.
- ^ "4-1 vs 4-2-1 Headers – What is the difference? What makes more power?". www.redline360.com. 19 October 2012. Retrieved 24 September 2019.
- ^ "Header Dyno Testing & Comparison, Tri Y vs. 4 Into 1". www.superstreetonline.com. Retrieved 24 September 2019.
- ^ a b "Exhaust System Technology: Science and Implementation of High Performance Exhaust Systems". www.epi-eng.com. Retrieved 25 October 2019.
- ^ "Firing Order Swaps". www.enginelabs.com. Retrieved 25 October 2019.
- ^ "Bill Sherwood's Engine Page - Exhaust". www.billzilla.org. Retrieved 24 September 2019.