I have a 2013 VW Jetta TDI sportwagon with a manual 2-clutch DSG gearbox.
Recently you recommended that when accelerating you should go gently with medium power. Used to be good advice with old V-8s and an automatic slushbox. Not true now with close-ratio computerized fuel systems. I am a professional race car driver/builder and have done a great deal of fuel consumption testing. The best fuel mileage is obtained by rapidly accelerating through the gears up to the highest ratio available. In urban areas probably a double overdrive. If you have a reliable computer you can test yourself. Example : 0-60 in 8 seconds at 10mpg versus a soft-pedal 0-60 in 30 seconds at 15mpg.
Then you coast at 60mpg to zero. I average 48mpg in town and 53mpg on the highway. Turbo-diesels coast using nearly no fuel.
Accelerating mass requires a considerable amount of energy…The amount of energy to reach a given speed is the same whether you do it fast or slow…The problem arises when hard acceleration forces the engine into a very inefficient mode…Thye fuel energy (heat) gets blown out the exhaust and dumped into the radiator…But like you said, turbo-diesels and multi-speed transmissions can eliminate most of these loses. Hybrid cars do even better, using energy recovered from braking to aid in acceleration…I’m waiting for a turbo-diesel hybrid! With these technologies, driving habits become less and less important as computers enforce fuel efficiency…
Heavy throttle pressure but early shift points that take the engine to almost lugging seems to get the best. Full throttle with shifts at redline are not going to get the best fuel economy. A 0-60 run in 8 seconds will never average 10 mpg, more likely 2 mpg.
Years ago, I read about a BMW study that said best fuel economy was with short shifting a manual at about 3000 rpm, with large throttle openings (to cut pumping losses), like @keith said. But that was before hybrids, I know my mpgs would drop a lot if I did that.
The main reason you want to short shift and not rev to the redline is because in the lower gears, a considerable percentage of the vehicles total kinetic energy is in the engine’s rotating momentum.
When I was in college, I calculated the percentage of total kinetic energy the engine’s flywheel had in each gear of a Harley Sportster basing it on the engine having a 28 pound crankshaft that had an overall diameter of 8 inches.
In first gear, it was something like 35% of the vehicle’s total kinetic energy was tied up in the kinetic energy of the rotating crankshaft. In fifth gear, it was only around 10 or 15% of the vehicle’s total kinetic energy, based on memory, I lost the exact numbers long ago and it was theoretical anyway based on an estimated flywheel rotating mass and an assumed rider mass.
So the less flywheel the engine has, the less the penalty for not short shifting will be and the more efficiently the vehicle will accelerate.
You might consider buying an “underdrive” alternator pulley at the speed shop, there’s a lot of rotating mass in the typical alternator especially when you consider it spins two to three times the rpm of the engine.
With a step less CVT transmission, the engine rpm is steady as you accelerate so you don’t waste energy accelerating the engine over and over again in each gear plus the kinetic energy of a highly revving engine can be recovered by the transmission gradually upshifting to an overdrive ratio at the end of the acceleration.
In my attempts to get better than average gas mileage, I have found very little mpg gain by accelerating ultra slowly, in fact, I find it can actually worsen my gas mileage. The kinetic energy of a car going 60 mph is exactly the same whether you accelerated to 60 in four seconds or whether you took all day to get there. You might as well give the car that energy at the engine’s peak specific brake fuel consumption point, within reason of course. If your engine is so powerful that this results in wheelspin, naturally that’s inefficient.
The OP has a diesel, so short shifting to reduce pumping losses doesn’t apply.
IMHO there’s more to gain by operating any machine gently for better durability.
Today I twice took my 2010 Insight (hybrid, CVT) on a 2 mile loop with two accelerations per loop to the speed limit of 45 mph. The first time, with “aggressive” (~2/3 throttle) acceleration, the computer said 60 mpg average. The second time with “mild” (~1/4 throttle) acceleration, the computer said 63 mpg average. Certainly not that scientific, but I conclude that aggressive acceleration gives me no mpg advantage (if anything, a penalty) and certainly causes more wear and tear on the drive train.
Bad gas mileage doesn’t come from accelerating incorrectly so much as it comes from accelerating all the time because of myopic driving. I can spot people who get rotten gas mileage from a mile back, their brake lights are lighting up every 15 seconds or so as they weave through traffic.
Accelerating as perfectly as possible only to throw all that kinetic energy away with your brakes at the end of your acceleration because you weren’t paying attention to the traffic more than two cars ahead of you still results in bad gas mileage.
The real key to good gas mileage is to drive as if your brakes don’t work.
Any engine in a late model car, gas or diesel, will coast with little to no fuel consumption. A small diesel like the Jetta TDI engine might not brake the car as much as a gas engine would.
I don’t have any experience with the new TDI’s but our 1978 Rabbit Diesel had very good engine braking, only put it to use when on slick winter roads. It’s one of the cars that mom learned how to shift by ear on. There were shift points on the speedo but she learned by sound where the best points to shift were. 46-48mpg every tank for years.
It’s not compression that gives you engine braking, it’s pumping losses. Two stroke engines practically freewheel when you close the throttle and every revolution has a compression stroke in these engines.
In the late '60s early '70s when two stroke motocross racing motorcycles really started to take over, racers found that they could give these engines “compression braking” by opening the compression release.