[anupmathur]

Originally Posted by vsathyap

....
....
.... but sudden braking like the one I experienced, I could barely lift my left foot off the ground and by then, the car had stalled. So, is this discussion even practical?

Engine braking is not used during urgent/panic stop situations; it is used when you know that deceleration is required and have the time to downshift. The advantages are that the wear on the brake pads is reduced, the steering control is better than being in neutral, and the car stays in the power band, should the need arise to accelerate again.

[Rehaan]

Originally Posted by redcruiser

how about the following situation.

Sounds fine to me.

The only issue I see is...

Originally Posted by redcruiser

5th gear .... bring the car to speed of 30kmph .... and then i downshift to 2nd gear

This means you'll be in 5th at 30 km/h.

Meaning, your RPM will be dangerously close to idle (depending on the car & the gearing), which in a modern ECU-equipped car means that there's a possibility that the ECU will inject more fuel just to prevent stalling.

While a firm foot on the brakes would mostly counteract this, I'm just bringing it up for discussion. A slightly earlier downshift, or a press of the clutch at the correct time would negate the issue

[Durango Dude]

If you want full braking power, my experience says don't press the clutch at all, let the car stall. If you want to ride smooth caress the brakes and keep your left foot ready to press clutch and down shift. I experienced brake failure recently and depended on engine braking for getting us home safely, it was an intra city, resolved by HMP, Chennai, for Rs.923-00. This all for a dry surface. One has to be double careful on a wet surface and when wading. Having ABS and TCS helps, that's the best part of owning an old Elantra it came with all wheels discs, ABS and TCS.

[sgiitk]

There are two things to it. While for an MT vehicle one should not declutch, one must remember most AT vehicles 'declucth' as soon as the road speed will indicate a lower rpm. This is the reason why the car should be kept in a 'low' range when descending. In the lower ranges some modicum of engine braking is there.

[VW2010]

Meaning, your RPM will be dangerously close to idle (depending on the car & the gearing), which in a modern ECU-equipped car means that there's a possibility that the ECU will inject more fuel just to prevent stalling.

Meaning, your RPM will be dangerously close to idle (depending on the car & the gearing), which in a modern ECU-equipped car means that there's a possibility that the ECU will inject more fuel just to prevent stalling.

I tried testing this. So in my MT Jetta riding in 5th gear without any accelerator input keeps the speed at around 50-55KMPH. There is no acceleration and the car RPM is around 930. The current fuel consumption reading actually shows around 45km/l. Shift to Neutral at the same speed the fuel consumption actually goes down to around 25-30km/l.

The car simply stalls if i reduce the speed to anything below 50 where the engine is unable to lug and the max lug it can take is between idle rpm and in gear rpm. In this case the idle RPM in my car is around 736 RPM. So the engine tries to lug around between 736 to 950 RPM after which the car runs on the available torque without any acceleration input. The fuel consumption during this attempt to raise from 736 to say 950 RPM is in line with Neutral fuel consumption only.

The ECU appears to try to keep the engine above an optimal RPM or simply shuts down and the fuel usage may be minimal considering the time it takes to shut down.

So driving in gear and letting the car coast on its gear and available torque is the best efficient driving method with zero acceleration. This though will shut down the engine in say an uphill road. On a flat road if you want maximum efficiency just go to your top gear slowly and let the engine run on its torque

Shifting to neutral only showed higher fuel consumption and hence staying in gears may actually help while coasting to a stop.

[VW2010]

This means you'll be in 5th at 30 km/h.

Meaning, your RPM will be dangerously close to idle (depending on the car & the gearing), which in a modern ECU-equipped car means that there's a possibility that the ECU will inject more fuel just to prevent stalling

More corrections from my side from the previous post. More test confirm few things

1) There is no way to simply drive at 5th and 30kmph without clutch usage or completely depressed clutch which means coasting. This is fine and the ECU does not try to do anything.

2) When you down shift to 2nd the engine RPM and car speed are pretty much matched that the engine does not hit the below idle rpm speed or lug

3) Shifting to neutral or staying in gears does not matter while slowing down. The engine consumption is the same - Correction from previous post.

4) Engine shuts off if one even simply tries to upshift say from 2nd to 3rd and release clutch abruptly without any fuel. Either the accelerator needs to be depressed or you gradually use the clutch to match the speeds.

5) Every shift up without acceleration say from first gear, use the torque to move and upshift takes the engine RPM way below idle RPM and this is where clutch comes into play and inturn the ECU trying to feed more fuel to keep the engine running. The engine RPM Goes way low upto 500RPM and takes lot of effort with clutch play to get this speed up and so does consumption of fuel when trying to achieve this.

6) Downshifting is fine as long as the gear you are getting into at the speed you are downshifting(in this case 30KMPH) is fine with the gear. In this case both 2nd and 3rd will probably handle fine with no major loss in fuel.

7) What to monitor is more than speed simply the engine RPM. If 30kmph at 2nd gear is above idle rpm, downshifting will even be smooth and fuel consumption hardly more than usual riding at that speed in the second gear.

Sorry about the earlier post. I will try to post a video with RPM, fuel consumption and some patters if it helps but this was fun.

[TheTeacher]

I have never ever pressed the clutch during braking; usually just brake or brake & downshift, depending on the circumstances. Of course I do depress the clutch just before engine stall.

But there's a video by Bosch of their ABS system in which they show the driver slamming both brake and clutch together, as though that is the correct technique. What gives?

[sgiitk]

@TheTeacher;I guess Bosch is showing the average driver, not a skilled one. The unskilled chap tends to jam on both brake and clutch at the same time. A good driver will never declutch until almost stopped/stalled.