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Redline

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This article is about engine speed. For other uses, see Redline (disambiguation).
Tachometer showing red lines above 14,000 rpm.

Redline refers to the maximum engine speed at which an internal combustion engine or traction motor and its components are designed to operate without causing damage to the components themselves or other parts of the engine.[1] The redline of an engine depends on various factors such as stroke, mass of the components, displacement, composition of components, and balance of components.

The word is also used as a verb, meaning to ride or drive an automotive vehicle at its maximum engine speed.

Variation of redline

The acceleration, or rate of change in piston velocity, is the limiting factor. The piston acceleration is directly proportional to the magnitude of the G-forces experienced by the piston-connecting rod assembly. As long as the G-forces acting on the piston-connecting rod assembly multiplied by their own mass is less than the compressive and tensile strengths of the materials they are constructed from and as long as it does not exceed the bearing load limits, the engine can safely rev without succumbing to physical or structural failure.

Redlines vary anywhere from a few hundred revolutions per minute (rpm) (in very large engines such as those in trains and generators) to more than 10,000 rpm (in smaller, usually high-performance engines such as motorcycles and sports cars with pistonless rotary engines). Diesel engines normally have lower redlines than comparatively sized gasoline engines, largely because of fuel-atomization limitations. Gasoline automobile engines typically will have a redline at around 5500 to 7000 rpm. The Ariel Atom 500 has the highest redline of a piston-engine road car rated at 10,600. The Renesis in the Mazda RX-8 has the highest redline of a production rotary-engine road car rated at 9000 rpm.

In contrast, some older OHV engines had redlines as low as 4800 rpm, mostly due to the engines being designed and built for low-end power and economy during the late 1960s all the way to the early 1990s. One main reason OHV engines have lower redlines is valve float. At high speeds, the valve spring simply cannot keep the tappet or roller on the camshaft. After the valve opens, the valve spring does not have enough force to push the mass of the rocker arm, push rod, and lifter down on the cam before the next combustion cycle. Overhead cam engines eliminate many of the components, and moving mass, used on OHV engines. Lower redlines, however, do not necessarily mean low performance, as some skeptics sometimes assume. For example, a Supercharged Buick 3800 V6 with a redline anywhere from 5500 to 6000 has a torque curve that peaks at 2600–3600 rpm, yet the engine is a strong performer from takeoff all the way through to the redline.

However, that an engine "pulls hard" all the way to its redline and running the engine to the redline before shifting does not automatically guarantee maximum performance and efficiency. Tachometer redlines generally indicate the maximum safe operating speed of the engine, rather than their peak efficiency and power production speed. Engine and chassis dynamometer testing nearly always reveals decreasing returns in torque and horsepower as an engine nears its redline, as volumetric efficiency and parasitic loads result in diminishing returns relative to fuel and air consumed. Engine builders and tuners strive for maximum area "under the curve", referring to the horsepower curve resulting from engine testing. Horsepower is the rate at which an engine is performing work rather than a measurement of an engine's actual power output at a given time, which is actually indicated by torque, or the twisting force available at the crankshaft. Torque is what performs the actual work, and high torque output over a large "power band" indicates better engine "balance" and tuning.

Engine builders, engineers and tuners read dynamometer results as a map of engine performance rather than a chart of data points. Since horsepower is calculated from torque and engine speed, at the point where torque output begins to drop and cannot be offset by increased engine speed, the engine is becoming less efficient and the horsepower output falls as a result. Beyond peak horsepower speed, additional engine speed only wastes fuel and increases wear and tear on the engine and other powertrain components. Horsepower and high engine speeds are grossly overestimated as indicators of performance in most applications, and high horsepower numbers that are dependent primarily upon engine speed rather than torque output have to be offset by increasing torque via torque-converter design, transmission gearing, differential gearing and tire size so that available torque is used and multiplied most effectively.

Typically this results in the need for lower gearing and additional transmission gear ratios and shifts to keep the engine operating within a relatively narrow power band. Additional complexity in the powertrain and gear reduction results in more parasitic load and higher power losses. The lack of multi-speed transmissions, the ability of the clutch to slip and gradually apply and the huge amounts of torque produced at low engine speeds are major factors in Top Fuel dragsters and Top Fuel Funny Cars being the fastest-accelerating and fastest piston-engined vehicles on the planet. They have a relatively low "redline" of approximately 8500 rpm, but do not reach "redline" during a 1000-foot dragstrip pass, during which they will accelerate from 0 - 300+ mph in just over 4 seconds. Torque output and the high efficiency of a direct-drive powertrain with gear reduction only at the rear differential make that performance possible.

While small-displacement, short-stroke automomobile engines are capable of very high engine speeds, motorcycle engines can have even higher redlines because of their comparatively lower reciprocating mass. For example, the 1986–1996 Honda CBR250RR has a redline of about 19,000 rpm. (Though due to regulations in Japanese motorcycle manufacturing this was later lowered to 18,000). That, however, is a two-stroke engine, and two-stroke engines are being increasingly replaced by 4-stroke engines due to efficiency, reliability and performance gains with modern 4-stroke technology. While 4-stroke engines of small displacement are capable of very high engine speeds, it is once again the lack of useful torque at lower engine speeds due to "efficiency" requirements mandating small displacement engines with supposedly reduced emissions that require high speeds.

An excellent example of mandating "fuel efficiency" and "low emissions" at the huge expense in resources necessary to design and develop engines capable of high crankshaft speeds to offset low torque output is Formula One. The redline of a modern Formula One car can approach 20,000 rpm, for example, although regulations in 2010 limit the maximum engine rotation to 18,000 rpm [1] in order to reduce vehicle speeds, engine failures, fuel consumption and expense. The engine speed limits were created after higher speeds led to frequent engine failures, high fuel consumption, more pit stops for fuel and more danger to car and crew during those stops, reduced overall race safety and decreased fan engagement and support. Prior to the engine speed limits, engine speeds reached over 20,000 rpm on the Cosworth engine.

Overall, reduced engine speeds has benefited the sport because there is more racing, fewer accidents and breakdowns and racing fans get more value for their time and money invested in watching races. Costs across the board have dropped, and competition has increased. Still, Formula One is very much out of step with most "mainstream" motorsports where larger dispacement engines and lower engine speeds dominate. When compared to much larger engines producing much higher torque output at far lower engines speeds and powering much heavier vehicles to similar speeds, the diminishing returns of very high engine speeds and small displacements become apparent. And while many other motorsports, such as NASCAR, feature racing with the engines and cars spending the majority of their events at high speeds, Formula One cars spend much less time at full vehicle and engine speed, and typically the maximum engine speeds are seen while downshifting going into and exiting corners rather than in "high gear" for lengthy periods of time.

Rev limiter and implementation

The actual term redline comes from the red bars that are displayed on tachometers in cars starting at the rpm that denotes the redline for the specific engine. Operating an engine in this area is known as redlining. Straying into this area usually does not mean instant engine failure, but may increase the chances of damaging the engine.

Most modern cars have computer systems that prevent the engine from straying too far into the redline by cutting fuel flow to the fuel injectors/fuel rail (in a direct-injected engine)/carburetor or by disabling the ignition system until the engine drops to a safer operating speed. This device is known as a rev limiter and is usually set to an RPM value at redline or a few hundred RPM above. Most Electronic Control Units (ECUs) of automatic transmission cars will upshift before the engine hits the redline even with maximum acceleration (The ECU in a sports car's automatic transmission will allow the engine to go nearer the redline or hit the redline before upshifting). If manual override is used, the engine may go past redline for a brief amount of time before the ECU will auto-upshift. When the car is in top gear and the engine is in redline (due to high speed), the ECU will cut fuel to the engine, forcing it to decelerate until the engine begins operating below the redline at which point it will release fuel back to the engine, allowing it to speed operate once again.

However, even with these electronic protection systems, a car is not prevented from redlining through inadvertent gear engagement. If a driver accidentally selects a lower gear when trying to shift up or selects a lower gear than intended while shifting down (as in a motorbike sequential manual transmission), the engine will be forced to rapidly rev-up to match the speed of the drivetrain. If this happens while the engine is at high rpms, it may dramatically exceed the redline. For example, if the operator is driving close to redline in 3rd gear and attempts to shift to 4th gear but unintentionally puts the car in 2nd by mistake, the transmission will be spinning much faster than the engine, and when the clutch is released the engine’s rpm will increase rapidly. It will lead to a rough and very noticeable engine braking, and likely engine damage. This is often known as a 'money shift' because of the likelihood of engine damage and the expense of fixing the engine.

Examples of performance automobile piston engines

Examples of production automobile engines

Piston

Car Engine displacement (cc) Engine type Redline rpm
Chevy Trailblazer 2776 I4 diesel 5000
Honda Amaze 1198 I4 8000
Suzuki Swift, Maruti Suzuki Dzire, Tata Indica 1248 1.3 diesel (Fiat 1.3 Multijet 75 PS) 6000
Tata Zest, Tata Bolt 1193 turbocharged I4 5500[5]
Toyota Etios, Toyota Etios Cross 1496 1.5 (2NR-FE) 5100
Toyota Etios Liva 1197 1.2 (3NR-FE) 7500
Toyota Etios 1364 1.4 diesel (1ND-TV) 7500

Rotary

  • Mazda RX-8 1.3L twin rotor 9000 rpm (rev limiter 9400 rpm), with the rotor making one revolution every three revolutions of the crankshaft (i.e. max 3000 rpm). Although the engine has no reciprocating components and could turn much faster, the speed has to be limited to avoid the risk of the clutch breaking up under extreme centrifugal force, which would be highly dangerous as parts could penetrate the passenger cabin.

Examples of motorcycle engines

See also

References

  1. ^ Car and Driver Magazine: Glossary of Terms - Redline
  2. ^ "Alfa Romeo Giulia 2.2 Diesel - YouTube".
  3. ^ http://www.corvettemuseum.org/specs/2006/LS7.shtml
  4. ^ https://media.ford.com/content/fordmedia/fna/us/en/news/2015/06/02/526-horsepower-ford-shelby-gt350-mustang.html
  5. ^ CarDekho: Tata Zest Expert Review
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