What is an engine speed cutoff and why it is needed
You can often hear from motorists how they rev the engine up to the cutoff, or their powertrain is bogged down by the cutoff. Basically, this happens when there is a need or desire to accelerate the vehicle to maximum speeds, or when the arrow from the tachometer enters the red zone at various speeds.
It is important to understand that the cutoff is not a breakdown or malfunction of the engine at all. It is a rather useful function incorporated into the car by the engineers. In fact, with its help, it is possible to additionally protect the power unit from excessive loads, and provide its work in optimal and safe modes.
In simple terms, when the engine reaches its peak load indicators, the cutoff occurs. It prevents the engine from cranking up even more as the engine is already overloaded. At such moments, drivers feel as the power unit does not pull, works with jerks, pauses in traction are observed.
What it is and how it works
As we have already managed to find out, the cutoff is a special, laid down by the engineers, protection for the engine, which limits the maximum frequency of rotation of the crankshaft. Because of this, the driver has an opportunity to increase revolutions only up to the foreseen standard cut-off, but not higher. It helps to save the engine and save it from extreme overloads.
That is the engine cut-off is purely a protective function, and not any malfunction, failure or infringement in operation of the internal combustion engine.
Now it is interesting to know how this engine cutoff works. In the case of modern injector engines, it looks something like this. When the engine reaches the limit set during production, the electronic control unit switches off or blocks the fuel supply to the working cylinders, which contributes to a decrease in the shaft speed.
Having already understood what the cutoff means and why it is needed in engine operation, it is worth talking about some equally important parameters. There are several characteristics applicable to the engine, which determine the value of peak load values that activate the cutoff. These take into account:
- the speed of the car;
- the current revolutions made by the crankshaft;
- Exhaust pressure in the engine with a turbocharger (turbochargers).
It is worth separately considering the existing varieties of cutoffs, which differ from each other on the principle of action, but have the same goal. It is to preserve the integrity of the engine and its individual components.
Speed
There is a cutoff, which is triggered when certain maximum speeds are reached. With this feature, automakers limit the maximum speed of their vehicles. Typically, the maximum speed cutoff is used in conjunction with cutoffs that respond to engine speed.
In the case of the speed cutoff, it is used so that the car could not accelerate beyond certain predetermined parameters. And the rpm cutoff directly protects the engine itself when driving at different speeds and gears.
If the vehicle is equipped with a speed limiter, the electronic unit also cuts off the fuel supply to the working cylinders when the maximum value is reached. This prevents the vehicle from accelerating further. As soon as the vehicle starts to drive within the set limit, i.e. the speedometer arrow falls below the peak, the ECU again issues a command and the fuel supply is resumed.
In practice it looks like this. The car accelerates to 250 kilometers per hour, which is the most common peak value for most modern cars with fairly powerful engines. At this point, the cutoff is activated. The driver simply cannot gain speed further because no fuel enters the cylinders. When the speed conditionally drops by 5-15 kilometers per hour, the ECU turns on the fuel supply, and the motorist can pick up speed again. But as soon as he reaches the peak value again, the ECU turns off the fuel supply again.
Such system is invented to prevent the driver from driving his car faster than the cutoff.
Some car owners are not happy with the fact that the speed of their car is artificially limited. The speed limit is also often referred to as an electronic bracelet. In fact, it is a program code that is entered into the controlling electronic unit. To remove these artificial restrictions, the electronic brain of the engine is re-flashed, getting rid of the limiter, which allows the car to go beyond the cutoff. To what extent this is necessary, it is difficult to say.
Turbine
There is another type of cutoff. It responds to the exhaust gas pressure in the turbocharger as well as the speed of the turbocharger. As you understand, such a cutoff is used only on turbocharged engines. And such a solution is considered more effective than a simple shaft speed cutoff. The point here is that the revolutions of the crankshaft itself may not yet reach the peak maximum values, that is, they do not threaten the integrity of the engine.
But these same figures become critical for the turbocharger. Operating under such loads can provoke serious negative consequences and lead to turbine failure. And this is a rather expensive element of a turbocharged engine, which needs to be taken care of.
The cutoff on turbocharged engines allows you to somewhat limit the speed of the turbocharger, which helps provide additional protection against destruction of the element.
Crankshaft
The most common and actively used cutoff variant is considered a limiter, which takes the crankshaft speed as a basis. That is, here we are talking about engine speed control. Here the cutoff has certain limits, above which it does not allow the shaft to rise.
Not only foreign cars are equipped with such limits on revolutions of the power unit. Many did not even know and did not guess, but also domestic cars are also equipped with cutoffs from the factory. The same Lada Kalina (Lada Kalina) is triggered at 6500 rpm. This is about the same value of the rev limit as is provided for the Korean cars Hyundai Solaris and Kia Rio, which are popular in Russia.
The 6,500 rpm value at which the limiter (cutoff) comes into play has become an established standard, which is rather conventional in nature. In other words, it is not the same and obligatory for all, but the cut-off at 6500 rpm turns out to be the most acceptable and suitable for standard engines with relatively low power and torque indicators.
At the same time, there is a wide list of car engines, where manufacturers provide completely different parameters of revolutions for triggering the cutoff. Just there are power units, which are initially positioned as high-speed, due to what in standard mode of operation their crankshaft rotates at about 7-8 thousand revolutions per minute. And it is not quite correct and correct to put a limit on a mark in 6,5 thousand revolutions per minute here.
In the case of sports cars, hypercars and other heavy-duty vehicles with high-performance engines, the cutoff is located at even higher values.
The cut-off is always activated after the appropriate command from the electronic control unit. It prevents the vehicle from continuing to accelerate if the crankshaft has already reached its limit, albeit artificially set.
Car manufacturers try to set the cutoff at a level at which the engine spins up to 95-100% of its maximum operating RPM values. Engineers try to leave about 5% as a reserve and reinsurance from the limiting values of revolutions, at which the engine is able to work without losses and serious damage to itself.
For example, if the power unit has a standard limit of 6.5 thousand revolutions, then in reality such an engine can revolve up to about 7 thousand revolutions, but without being at its peak loads, contributing to destruction, rapid wear and the occurrence of breakdowns. Such reserve allows to create optimal functioning conditions, prolonging the service life of the power unit.
The RPM limit at which the cutoff will be triggered is easy to determine yourself. To do this, it is enough just to look at the dashboard. And to be more exact, on your tachometer. The line, whence the white stripe changes to red, is the very limit. When the driver brings the revolutions to the red tachometer line, the cutoff is activated, the ECU gives a signal and the fuel supply to the cylinders is stopped. As soon as the tachometer arrow goes down to the white zone and the revolutions of the crankshaft correspond to the values programmed in the ECU memory, fuel will be fed to the working cylinders again.
Useful Tips
It is worth noting that modern cars are actively equipped with various limiters. And quite often you can find several different cutoffs on the engine used simultaneously. This applies to boosted and powerful atmospheric and turbocharged engines.
The engine speed limiter is objectively more useful than harmful. It is difficult to name the limiter as some disadvantage, as in fact it does not limit capacity or dynamics of a vehicle. But it allows keeping the engine intact, preventing premature wear and protecting it from serious damage caused by overloading.
Many people wonder if it is allowed to rev their engine up to the cutoff, and if the engine itself will suffer from this. It is allowed to do this. And here we are not talking about any threat of damage to the power plant. Simply do not go beyond a red line on a tachometer.
There is nothing good in frequent use of the car at high revolutions, when the tachometer needle is constantly in the red zone. It promotes reduction of engine life and occurrence of those or other malfunctions, caused by excessive engine loads.
But the point is that there is no need to drive revolutions up to the red zone. This does not increase the power or maximum speed, but only decreases it. The engineers have conceived so that the engine reaches its maximum power exactly at certain and sufficiently high crankshaft revolutions. This is about 90-97% before the cutoff occurs. For example, let us assume that the car has a revolutions limiter at the standard mark of 6.5 thousand revolutions per minute. It means that you will manage to reach maximum capacity of your engine at about 6-6.3 thousand revolutions per minute. That is, you have not reached the cut-off point, but have approached it by a small distance.
As soon as you go beyond the cutoff, the power will immediately fall, the speed gain will stop, because there is no more fuel flowing into the cylinders. You will only try in vain to squeeze the gas pedal, destroying the pedal assembly itself and provoking additional trouble with your completely unnecessary and unjustified actions.
If you want to receive the maximum efficiency from the engine, to feel its productivity, keep revolutions near cut-off, but do not pass this edge. Motor operation in such mode is absolutely safe, there is no risk of damage or excessive wear from overloading. Now the engine is running in the optimum mode, for which it was originally adapted by the designers.
When performing engine tuning, the cutoff limit values are sometimes shifted upwards to increase the engine output. Sometimes the limiter is turned off completely, giving access to the full power and maximum speed the car is capable of.
Blocks where the cutoff can be shifted theoretically give a boost in power and increase top speed. But such tuning increases the load on the engine, which begins to wear out and fail faster. In order to switch off the cutoff or shift it, you need a specially prepared car whose engine structure uses strong, wear-resistant and reliable components capable of withstanding the increased loads. Otherwise, you will only do harm to the engine, provoking a significant reduction in its service life.
As to usual engines, it is strongly not recommended to switch off limiters on them. If you do it and twist the engine above the limit values established by designers, failures will begin to occur extremely quickly. It is possible to extract at most the same 5% margin. In fact, often the cut-off is triggered at 95 % of its maximum working revolutions. 5% is needed purely for reassurance. Such a shift of the cutoff does happen without significant risks, because then the values do not exceed their limit anyway.
But attempting to move the cutoff to more than 5% is strongly discouraged. This will lead to unpredictable but surely negative consequences.
Finally, it is worth giving one simple advice to all those who like to operate the car at high revolutions. Such conditions lead to increased consumption of not only fuel, but also of oil. Therefore, it is important to constantly monitor the amount of lubricating fluid in the engine, replenish it as necessary and change it in a timely manner. Such operation reduces the life of the oil, plus puts higher demands on its quality. At high, constant speeds it is critical to use high quality lubricants to minimize engine component wear and protect the engine from possible serious breakdowns.
Engine RPM limiter operating principle and types
Here is how the rev limiter works, and why it is important even for professionals
An engine needs a rev limiter to prevent damage to motor components from exceeding the speed of its mechanisms. But what are the different ways that the so-called “cutoff” works, and what can happen if your car doesn’t have one?
See also: The most expensive car breakdowns
So, the “cutoff”, aka maximum speed limiter. According to the name, its purpose is also clear – the system limits the maximum speed of revolutions that the engine crankshaft can reach, thereby preventing increased wear or breakdowns. Especially fairy look “explosions” of diesel engines – because of the heavy pistons there is a breakage of connecting rods, and they break through the engine through, demolishing the “head” of the engine or the debris of the connecting rod punching the engine block. There is no cure for such automobile injuries:
Any engine is designed to function within a certain limit, which is measured in crank rod revolutions per 1 minute.
The RPM limiter is installed on many cars from the factory, but it definitely should not be confused with the red maximum RPM zone. The two are not always the same. The red zone is a portion of the engine’s operating RPM. Staying in this zone should be as little as possible, and nothing serious should happen to your engine, except increased fuel consumption and low productivity, but it is the limiter that should stop the set speed of rotation beyond what the engine elements are physically able to withstand.
What types of limiters are there in modern cars?
RPM limiting with a Valvetrain system
One example of a working system is Valvetrain technology. Most such designs use metal springs to actuate the valves and return them to their designed, correct position, even when used hard. But if the engine is cranked too fast, the amplitude of the valve springs can go beyond their limits, effectively leaving the valves open.
This phenomenon is called “incomplete valve closure,” which is not good. Misalignment in operation can result in decreased compression, ignition skips, or even pistons hitting the valves. After excessive “desynchronization,” ordering fresh parts for the engine will be very likely.
The second breakdown problem we have already described above is a broken connecting rod. Alas, it can also happen on gasoline engines when overspeed, but it happens more often on diesel engines.
Types of engine cutoffs by smoothness of operation
There are also such concepts as “hard” and “soft” limiters. “Hard” limiters are the ones that you “crash into” during intense acceleration with an unpleasant squeal, when the engine power suddenly disappears. “Soft” limiters slow the revs down gradually, it’s not as noticeable.
Sometimes, if there is a soft limiter on the car, a car owner may even wonder: is there one on this engine at all?
Let’s put it this way: if there is any doubt, whether there is a “cutoff” or not, do not play with fire, do not exceed the set revolutions in the red zone. Better yet, look in the datasheet to see what the manufacturer’s RPM limits are for your engine type. Always keep this number in mind and do not go even close to it, if you do not want to leave for sudden service by the tow truck.
“Soft” rev limiter
You could argue that a “soft” limiter is better for road cars because it puts less stress on various drivetrain components, and if reached, it will keep engine RPMs at a given maximum, without bouncing back a hundred or two RPMs down.
A “hard” limiter, on the other hand, allows you to get to the maximum up to the limit without going over it. One revolution of the crankshaft is not enough to go over the critical limit, which is the preferred option for racing cars.
“Soft” limiters gradually reduce fuel delivery, slowing and eventually stopping engine acceleration.
“Hard” rev limiter.
“Hard” maximum rev limiter cuts off either the spark or, more often, the fuel, stopping combustion and instantly robbing the engine of all its power. This is exactly what creates such an obvious effect. Combustion in the cylinders will be restored as soon as the rpm drops, then everything repeats.
The third type of maximum RPM limiter is “variable”.
“Variable”, “adjusting”, “changing” rev limiters are also not uncommon these days. They usually set the maximum RPM lower when the car is idling or when the engine is in the warm-up phase. Otherwise, their purpose is identical to the previously described options – not to kill the engine.
Warning!
It is necessary to strike a note to “racers” that if you approach a cutoff, say, in the second gear and want to pass to the third, but instead of it you “stuck” the first one, even the limiter cannot prevent sudden destruction of the engine. Rpm will jump over the limits, and anything can happen to the engine at this moment! Well, unless, having realized a mistake, you do not switch off a clutch in a split second. In general, be attentive!
