Clutch in a gasoline car VS Clutch in a Diesel car

diesel
gasoline
clutches

#1

I live in Spain now but grew up in the US.
I learned to drive stick with my dad while listening to Car Talk.
I’ve always driven gasoline cars until 2 years ago when I bought myself a Diesel Citroen Gran Picasso.
All good up to there.

Now my wife is learning to drive and here in Spain there is no such thing as a “learner’s permit” so you have to do any practice driving with an instructor and paying 25€ per 45 minute class.

As this is all very exciting for my wife, we talk about her experience every night when I get home and I try to explain my point of view on some things.
So last night we were talking about how to get the car started from a stop and how to use the clutch and accelerator and how to move your feet.

I said:
“With the car in neutral and with your right foot on the break, push the clutch in with your left foot and start the car. Now, with the engine started, put it in first gear and as you lift the clutch up you give just enough preasure on the accelerator so that it doesn’t stall and your on your way. Then the rest of the gears is a dance with your feet, pushing the clutch in while you take your foot off the gas, change gears and then let up on the clutch while you accelerate listening to the engine to know how much gas you need to give it”

So she goes to class today and talks to her instructor about this who says:
“With a diesel it hurts the clutch if you give it gas while your foot is on the clutch. You should always start in first gear by just slowly taking your foot off the clutch and the car will start moving. Once your foot is completely off of the clutch, then you should start to give it gas.”

My reaction was that this is impossible. Every gasoline car that I’ve ever driven would have stalled doing this but today at work when I went on lunch I tried what the instructor said: I started the car, put it in first gear and lifted my foot slowly off of the clutch without giving any gas at all expecting the car to stall. To my surprise the car started moving forward without the slightest hint of wanting to stall. It was mind blowing since I had assumed that my new diesel car would drive itself more or less like the gasoline cars I’d had in the past. After all a clutch is a clutch, the clutch mechanism isn’t different, it’s the method of combustion that’s different… I think.

So according to my wife’s instructor, In my diesel car, when I step on the accelerator just a little to get the car started in first gear I’m hurting the clutch.
I don’t have access to a gasoline car any more but I swear I had to gas it a bit when getting started in first gear and that was how my dad taught me.

I’ve done a lot of reading on the internet and found several articles about “riding the clutch” but I don’t ride the clutch, this is different.
I don’t drive with my foot on the clutch, always to the side except when I change gears, I don’t stop at a light and leave my the clutch pushed in while waiting and I don’t leave the clutch pushed in when waiting on a hill.

Does anybody have any thoughts about this?

Should i start off in 1st gear and just lift my foot off the clutch waiting until the car is actually moving before i accelerate?
I’ll change habits if it really is healthier for my car but it wont be easy.
It seems to me that just lifting your foot off of the clutch without giving any gas is hurting the car and not the other way arround but I don’t have any facts to back that up with, just a gut feeling.

I’ve had the car 2 years now and it’s seems to be fine but if there is anything i can do to prolong the life of the clutch…


#2

A diesel has more torque and at a lower rpm than the equivalent size gasoline engine, as you discovered when you tried the new to you technique.


#3

Agreed with oldtimer 11.

One of my antique motorcycles is a big inch flathead. (gasoline of course) Those engines are low RPM, high torque “grunt” motors similar to a diesel.

Many years ago I changed the transmission countershaft sprocket to a larger one; something to the tune of a roughly 17% overdrive so to speak. The guy at the bike shop was skeptical the old dog would even pull itself away from a stop light without bogging and dying.

It did not affect it at all. The bike will not only pull away from a light easily but it will do it in 3rd gear without hesitation. The bike still has the original clutch that was in it when I got the bike in 1974. There’s a lot to be said for mule power… :slight_smile:


#4

You are correct that the clutch in gasoline and diesel drive trains are the same except that the diesel clutch has to be sized to handle a higher torque for the same horse power.

What is different is the metering of fuel and air. With a gasoline engine when more torque is required, more air has to be admitted. As a result the fuel metering unit injects more fuel to make the mixture stociometric. With Diesel engines air is not metered. Diesel engine speed is controled by restricting the amount of fuel injected into esch cylinder at combustion. Without some sort of speed control, any excess injection would cause engine runaway or any lack of injection would cause stall. A lot of injection pumps control the injector racks with a centrifugal governor. When you want to increase speed the ‘throttle’ pedal adjusts the conter spring on the governor. So at idle as you engage the clutch; the governor senses the engine slowing; and injects more fuel into each cylinder to hold the speed constant. You can get to full rack i.e. full torque, at idle without touching the accelerator.

I have not driven a Diesel automobile to verify the following supposition./ Observe the follwing on a GM or Mercedes Diesel engined car… While cruising at a steady speed, can you just hold the accelerator at a constant position i.e. can you sense the governor holding the engine speed constant? When you are on cruise control, can you feel the servo moving the accelerator?


#5

It won’t hurt the clutch to give it a little gas as you’re engaging it. It’s even necessary. The puzzle that she’ll need to solve for herself is “how much”. It doesn’t take much, and it’s possible that the instructor was reacting to excess gas applied. Without seeing her drive, it’s impossible to second guess the instructor.


#6

To @ELAMERICANO
I think I have had the same experience as you as far as gasoline engines are concerned. When I learned on an underpowered 4 cylinder stick many years ago, the “dance” between the clutch and the accelerator was unavoidable. There was no way to only let out the clutch to get the car moving.
But in my stick today (in a 2008 model) I can let out the clutch slow enough on a flat road to get the car moving before applying gas. My understanding is that the computer engine control can detect the near stall condition and automatically apply a little more gas to prevent the stall. This only works with very careful clutch movement and so it is still possible to stall the car.
I have no idea if this is common in manual transmissions cars today because there are so few to try.


#7

Along with low end torq and final drive gear ratio the weight of the flywheel can have a significant effect on initially getting moving with a manual transmission. Old air cooled VWs were very forgiving to new drivers and their horsepower and torque were both anemic but the flywheels were quite heavy.


#8

With care one can get a gas powered car moving the same way. Just easier with a diesel. I’d give the diesel some throttle if I was driving.


#9

From a red light if you take the time to ease out the clutch to get the car moving before you apply the acceleration pedal you are going to have a bunch of irritated people behind you.

Just because someone has a job as a driving instructor does not mean they should have said job.


#10

Diesels have idle governors that automatically cause the injector pump to deliver more fuel if the engine slows down. You don’t need to give it the gas, the engine does it automatically in response to the additional load.

Governed gasoline engines act that way also, (farm tractors etc.) The “throttle” on a farm tractor does not directly open the throttle, it sets the speed the governor holds the engine at. You let out the clutch and the engine automatically opens its throttle in response, you disengage the clutch and the engine automatically closes the throttle to prevent the engine from running away.

Your driving instructor was correct.


#11

I agree w/the instructor also. Especially with diesels, best to be conservative w/ the gas pedal when letting out the clutch.

You wouldn’t think it would make that much difference as far as the wear and tear on the clutch. But when the instructor says it can damage the clutch if you apply too much gas, what he means is the friction of the clutch spinning against the flywheel doesn’t just wear a little more material than normal from the clutch surface, if aggressive enough it can cause the clutch material to overheat and then melt. A melted clutch is something you definitely don’t want.

About the required foot action: I drive my manual xmission Corolla here in San Francisco, a city which is very hilly. And the hills are steep. It’s hard to start the car moving from a standing start on a steep hill. One thing that makes this much easier is using the hand-lever emergency brake. I set it, then I don’t have to worry about the car rolling backward, so I can just concentrate on the left foot on the clutch pedal and the right foot on the accelerator, no worries about the brake pedal. Of course I have to release the emergency brake while I do this, but that I can do with my hand. If your car sports a hand-lever emergency brake, make sure your wife understands that’s an option too. Simplifies the foot action a little.


#12

I have driven a couple of diesel powered pickup trucks with manual transmissions, and you can engage the clutch quite aggressively on an idling engine without stalling it.

An idle governor on a diesel is a practical necessity. The high intake manifold vacuum of a gasoline engine acts like a limited idle governor. If the engine speeds up, the vacuum goes up, (manifold pressure goes down) and as a result, the cylinders take in less fuel/air mixture and that reduces torque which tends to make the engine slow down. As a result, the engine speeds up a little but does not run away when you take the load off the engine. If you increase the load, as in engaging the clutch a little too fast, the slowing down of the engine causes the intake manifold’s vacuum to go down (manifold pressure increases) and that results in an automatic increase in torque as a response.
In a diesel, there is no such automatic reaction to increasing and decreasing loads because there is no manifold vacuum, so without an idle governor, the engine would not have a stable idle speed. The oil warming up or the alternator finishing with the battery charging would make the engine race if there wasn’t a governor controlling the idle rpm.

Think of a governor as a cruise control for engine rpm.


#13

“With a diesel it hurts the clutch if you give it gas while your foot is on the clutch. You should always start in first gear by just slowly taking your foot off the clutch and the car will start moving. Once your foot is completely off of the clutch, then you should start to give it gas.”

One thing is for certain…the driving instructor is a moron.


#14

WOW, you guys are amazing. I’ve never had so many responses to a post in an online comunity like I have had here to my question. I’ve never had such fast responses either. I’m going to print this up since it is too much to read while at work and will respond to your questions shortly. Thank you again to everyone for the wealth of information, comments and personal experience.


#15

I’ve driven a couple of small diesels in Europe, there was no need to ‘baby’ the clutch, I drove it just like a gas powered car. Small diesel engines (that’s what they put in cars) will not damage the clutch if some throttle is applied when starting out.


#16

I agree with your wife’s instructor as to not needing to give it gas, but I also don’t think you are doing any significant damage by giving it a little throttle. It does not matter whether its a gas engine or a diesel, the more throttle you give it, the harder it is on the clutch. The damage or wear increases exponentially though, a little extra throttle, very minimal damage, a little more throttle will add more wear, full throttle all the time, the clutch will wear out much quicker.

In other words, lets say that 5% throttle takes 0.000001" off the plate, 10% throttle might take 0.0000015", 20% throttle might take 0.000005", 50% might go 0.0001" and 100% might take 0.001" per launch. Clutch wear goes up faster than the % throttle.


#17

In the early days of NASCAR the low torque from racing cams conbined with light weight flywheels that improved throttle response resulted in pit crews pushing the cars up to walking speed before the driver released the clutch. All that horsepower and the car couldn’t pull itself off of its own shadow.


#18

Could it be that the idle is just set a little higher on a diesel? In my Olds diesel, it wasn’t my experience that it had a lot more torque at low rpm. The thing could barely get out of its own way off the line. Once it was moving it was ok. When I first got it we pulled a camper down to Florida and I thought something was wrong with it. It was more like driving a four cylinder. Pedal to the floor and not much would happen. Maybe this was not typical diesel and it was a long time ago so maybe my memory is fading.


#19

Those old automotive diesels (non turbos) were slow dogs.


#20

Engine idle speeds were not meant to get a vehicle moving in a forward or reverse direction. I sure wouldn’t want to be behind that driving instructor during rush hour traffic and a dozen stoplights. Someone (not me) would probably drag him out of his vehicle and tell him what was wrong with his driving habits.