When you dip the clutch in a car, the revs will drop. This is due to the inertia of the flywheel, and the lack of air going in the intake as the throttle is release.
After the gear change is over and the clutch is released, the engine speed will go back up again, meeting the transmission speed. In extreme cases the engine speed will even rise slightly when the engine and transmission are disengaged. This creates a horrible jerk as a braking force occurs when the powertrain and transmission are reengaged and forced to spin at the same speed.
The cause of rev hang can be put down to the war on emissions. Recently, manufacturers have been intentionally implementing rev hang within ECU programming to save themselves from a potential emissions scandal. When the throttle is released and the clutch is dipped, an ECU with rev hang is programmed to leave the throttle slightly more open than usual. This strategy is used because it was found that suddenly closing the throttle produced a sudden spike in gas pressure within the crankcase which emphasised the rate of oil vaporisation, thus adding to emissions from the engine.
Also, when the throttle is suddenly shut, the fuel mixture suddenly becomes lean which creates NOx oxides of Nitrogen which also contributes to emissions. By keeping the throttle open for a small period of time after the pedal had been released, this pressure spike was avoided and cleaner emissions were achieved. These days car ECMs are even coded to individual vehicles by their VINs vehicle identification numbers and this makes them non-transferable between vehicles.
It is worth noting that ECM control of the combustion process has brought motorists many benefits cheaper unit cost, lower emissions, reduced fuel consumption, improved driveability, no need for regular tune ups and improved reliability, to name just a few but it also gave rise to features like the rev hang.
This issue is caused by what one major car maker describes as "non-linear throttle control". It is illustrated in the picture below. What it means for the car driver is that the ECM controls how the car engine runs, or more accurately how much the throttle valve opens, without full regard for the driver's commands issued through the throttle pedal. For example, when the driver floors the throttle pedal, the ECM directs the engine to increase its speed at a more leisurely pace and the same thing happens in reverse when the driver lifts his foot off the throttle pedal and the ECM directs the engine to keep running under power for some time longer.
Apart from the exhaust emission compliance issue accounting for the presence of the rev hang, a small degree of it may in fact improve drivability of cars with manual gearboxes by reducing the amount of drive-line snatch. Misleading information in other Web sites. Flawed explanations appear on many bulletin boards and Forums, asserting either that the purpose of the rev hang is to "burn unburnt fuel" or that the rev hang is caused by a slow reaction time of the analogue throttle valve.
Sometimes even "a heavy flywheel" is blamed It is beyond comprehension how could burning of unburnt fuel be possibly assisted by adding even more fuel after the throttle was closed.
As for the slow throttle valve, in EFI engines it only controls supply of air to the engine. Throttle hang would not function without injection of fuel and the fuel supply to injectors is capable of being controlled instantly, if it was intended by car designers.
So much for posts by Internet experts Which vehicles are affected? Many modern motor cars and possibly also motor cycles fitted with 4-stoke spark ignition combustion engines with electronic fuel injection and drive-by-wire throttle.
Diesel engine cars are not known to be affected. Perhaps not surprisingly, it is often lower-end models suffering most, as against more expensive models.
With upmarket cars more time and effort is being spent on designing solutions to avoid such a problem and car buyers are charged premium price for the extra effort. Also, buyers of expensive cars understandably have higher expectations and would not accept explanation that "rev-hang is a design feature of new vehicles". Cars fitted with automatic transmissions suffer the same rev hang problem as cars with manual gearboxes but drivers are much less aware of it because of the design and function of the auto 'boxes, especially those fitted with torque converters.
What is the effect of rev hang? Basically the problem for drivers used to driving cars built before the rev hang existed circa is one of annoyance and mental adjustment when driving new cars with rev hang. In the old days, we used to accelerate in low gear, back off the throttle, hit the clutch, select the next higher gear, release the clutch again and apply power, all within about 0. Cars with the rev hang require a forced pause between backing off throttle and depressing the clutch to change gears.
If the pause is not allowed, depressing the clutch immediately after backing off the throttle results in clearly perceivable increase in the engine speed at the time when it is least needed. This is caused by the ECM induced slow drop of revs and is not helped by the fact that disengaging the clutch too early lifts the load off the engine, until then working hard to drag the mass of the car forward. Clearly, the engine revs would shoot up when load is lifted off the engine that is still being operated under power by instructions from the ECU and not from the right foot of the driver.
Recordings of rev hang in action YouTube is full of videos showing tacho needles of cars being revved when stationary. After throttle was closed quickly the videos showed the tacho needle hovering at high readings before slowly returning to idle. Such videos are quite illustrative but of little value for evaluation of the effect of rev hang considering that the tested car was stationary and engine was not under load.
Proper demonstration of rev hang should be recorded when the test car is being driven, best at the point of gear change, like the one below: Toyota Yaris without QREVS operating another video similar to the one shown at the top of this Web page : movie file 2. Similarly, when changing from 2nd to 3rd gear at RPM the engine speed did not drop for 0. These behaviors and body motions put the Golf one step closer to the beloved first generation GTI I drove earlier this summer.
Furthermore, I found the 1. Rev hang aside, the six-speed manual is an absolute gem. Top it off with a fairly small footprint, a comfortable interior and plenty of utility and it proves to be a great all-rounder. And the added engagement of the manual transmission is the lynch pin of the whole package. Love them. Cherish them. Buy them! Don't let rev hang stop you from owning the most engaging transmission or keep you from signing up and leaving comments below.
Car Life. Type keyword s to search. Today's Top Stories. View Full Gallery. Robin Warner. Adapt to the needs of a manual transmission and reap the rewards of engagement.
0コメント