View Full Version : Diesel engine speed advice please
04-12-2009, 06:53 PM
I used to have a Civic petrol and a SGII and was surprised to find that driving at 30mph in 4th gave slightly better FE than 30mph in 5th and the rule of thumb "Lowest engine speed possible" doesn't really apply. Now I've got a '01 diesel Astra, and doesn't have an OBDII port so I can't use the SGII to work out what engine speed is the most efficient. The car goes 750 miles between fill-ups so it's hard to figure out small changes in such large distances. The engine is a turbo, and I think it kicks in at 2,000 rpm. Torque peaks at 1,800 rpm giving 165 N.m (122 lb.ft). I don't know whether the simple rule of thumb of keeping the engine speed as low as possible applies to diesels, or the engine speed should be kept somewhere halfway between 1,000-2,000 rpm.
Anyway, for each road speed, I've listed the engine speed in each gear - is it reasonably easy to suggest which gear I should use in each speed or it is impossible to know without the SGII - i.e. I'm asking a "how long is a piece of string?" type question?
35mph in 3rd gear - 2,100 rpm
35mph in 4th gear - 1,400 rpm
35mph in 5th gear - 1,100 rpm
45mph in 4th gear - 1,800 rpm
45mph in 5th gear - 1,500 rpm
55mph in 4th gear - 2,300 rpm
55mph in 5th gear - 1,750 rpm
Any advice would be much appreciated!
___30 mph in fifth in a std. Civic was below its lug point more than likely. Around 1,300 - 1,400 RPM should prove to be a great RPM in which to hold high FE and still accelerate slowly away if needed.
___Regarding the diesels, the 2.2L iCDTi I-4 loved the lower stuff and would show max at around 1,200 to 1,500. The 335d's 3.0L I-6 I have now will run at 50 to 55 mpg at 1,200 to 1,450 RPM in fourth, 1,200 to 1,600 in fifth and sixth.
___I hope that helps?
04-12-2009, 09:24 PM
Every engine will have a different BSFC map because of different fuel systems, air systems, etc. but the following trends will (I think) hold true.
1) Best bang for the buck (power/fuel rate, or BSFC) comes with engine speed near peak torque, and something like 70-80% load. But this speed&load combo will give you way more power than you need for cruising.
2) When you don't need any power from the engine, the lower the engine speed the better (0rpm is best, low idle is second best)
3) When you need power somewhere in between 0 and the power you get in #1 above, you'll get that power most efficiently at some speed in between low idle and peak torque. The more power you need, the higher the speed that gives it to you the most efficiently.
4) There will be some engine speed that is "too low" for efficiently providing even a modest amount of power. What that speed is probably depends a great deal on the engine. I'm guessing here, but you might get an indication of a usable low speed from looking at the torque curve.
My truck has peak torque at 1800 rpm too, and I find that I tend to do pretty well cruising with the highest gear that will keep the rpm's at least 1200-1300. However, when hills or acceleration require more power, I think I get that power more efficiently at maybe 1600-1900. The torque of my engine drops off dramatically below about 1500 rpm.
04-12-2009, 11:04 PM
So to summarise, I should cruise at the lowest engine speed as possible, i.e. 5th gear all the way down to 900 rpm. When accelerating, I should be keep the rev band at about 1,600-1,900 rpm and not let the turbo kick in?
Thank you very much to the both of you for your time, I really appreciate it!
 I did that once, the engine seemed to be OK with that, just rumbled slightly, I moved down a gear only because I wasn't sure if it was efficient to cruise at 900 rpm.
 Going from 3rd to 4th is a little difficult as the gap between the ratios of those 2 gears is quite significant on my car, so I have to shift into 4th at about 2,200 rpm.
04-13-2009, 09:43 AM
900 rpm is likely too low. Wayne is right about using the 1200-1600 range - go with that.
04-13-2009, 10:10 AM
In my 06 VW TDI, the max tourque is at 1750 RPM. That equates to about 55MPH in 5th gear. I try to take the hills at that speed, but on the flat surfaces, I like to Pulse and Glide (NIce-on) in the 45 to 55 MPH range which is about 1200-1400 RPM at the low end. This will consistently give me in the 70 MPG range unless their is head winds. My TDI will lug if I get below 1200 RPM.
Below are some pics of a recent trip from just south of Bowling Green OH, to Cincinnati, OH, Hope this helps. Sorry for the low quality pics, but my cell phone was all I had. I think that you will have find out what works best for your diesel by trial and error.
04-13-2009, 10:23 AM
Brilliant thanks very much for all of your help! Much appreciated.
From now on, I'll be driving in 4th gear for 35 mph, 5th gear for 45 mph and 55 mph and upwards.
Changing the subject slightly - as you talk about P&G with NICE-on - I assume it doesn't make a huge difference for diesels whether the ICE is on or off when gliding? My Astra has much less rolling resistance than the Civic, so I'm thinking about trying it occasionally. Yesterday when driving home, I glided in countryside lanes for about a mile at 45 mph losing only 5 mph, mainly through braking as there was a slow moving car in front of me driving at 40mph in a 60mph zone. However, the engine is very weedy so hard to pulse for a short period of time.
04-13-2009, 12:45 PM
.... as you talk about P&G with NICE-on - I assume it doesn't make a huge difference for diesels whether the ICE is on or off when gliding? .....
I would much rather to a NIce-off glide, but my year TDI has ceramic glow plugs and they tend to have trouble. So I keep their use to a minimum. Also, there are issues with turbo cooldown and the turbo and oil heating up due to turbo temperature. That is why I do mostly Nice-on glides.
04-13-2009, 03:59 PM
I agree w/ Bestmapman - deep thermal cycles and hot shut downs can be hard on a variety of things. You can buy a lot of fuel for the price of a new turbo (or glow plugs, or injectors, or ...) Some might get away with it, but I'd rather play it safe with mine.
04-14-2009, 11:00 AM
Same here. I hardly ever Nice-off unless i see just-gone-red lights at the bottom of the hill.
04-14-2009, 01:37 PM
Add me to the list of drivers that are mainly NICE-on. There are a few places where I will FAS such as if it's an extended glide down a hill or a coasting up to a long red light. Diesels idle very lean so luckily, fuel economy doesn't suffer as much doing NICE-on with a diesel as with a gas car. Some very preliminary testing on my car leads me to believe my glow plugs aren't used when coolant temperature is above 25C, but I will have to do more to confirm that.
Back to the question at hand, I try to stay above 1200 rpm.
04-15-2009, 09:18 PM
Some very preliminary testing on my car leads me to believe my glow plugs aren't used when coolant temperature is above 25C, but I will have to do more to confirm that.
My car's instruction manual says that the glow plugs aren't used at all unless the external temperature is below a certain point, but it doesn't say what. I will know by the glow plug warning light - if I turn the ignition on and the glow plug warning light doesn't come on, then obviously means the glow plugs aren't being used and I can immediately start the engine. I've never seen the glow plug warning light come on at any time and I've started the engine when the external temperature display displayed 5 deg C (40 deg F) but I haven't had the car long enough to know what external temperature it has to be for the computer to enable the glow plugs. My point is, does your manual say the same thing?
Naturally, as I'm new to the diesel engine area, I might be wrong - the glow plug warning light probably is only there to make sure I give the engine time to warm up, and the glow plugs do actually come on until the coolant temp goes above a certain temp, like you say. What's your comments on my thoughts? Do I have the wrong idea or...?
Thanks for your advice btw, much appreciated!
04-16-2009, 11:40 PM
It sound like you have the right idea about the light being there to make sure you give the plugs time to warm up. I don't have my owner's manual here, but from what I remember, the glow plugs in my car are used to help with cold starting when the coolant temperature is below 9C. Above that, the indicator light goes out immediately, but I believe the plugs are still used to some extent. I'm not sure if they come on when starting, but they are definitely on for a bit while the engine is running. The reason for that is to improve combustion quality and lower emissions. With a different car and different emissions regulations, your glow plugs may behave differently than mine.
The reason I feel the plugs are on for a while after the engine starts is that I hear my glow plug relay once in a while. It clicks off when the engine goes above 2500 rpm and clicks back on when the revs drop again. A few times recently, I've been idling and heard the relay click, accompanied by a momentary change in engine sound. When that happened, I took a quick look at my ScanGauge and it indicated 25C coolant temperature. After warming up beyond 25C, I don't hear the relay again.
Here's where it gets complicated and I need to do more testing: My car has two sets of glow plugs. One set is in the combustion chamber to help with cold starting and emissions, and the other set is in a coolant pipe to help my coolant warm up faster. I don't know whether all of them or only one set is being activated. I should also verify whether the plugs come on for starting above 9C when the indicator light goes out immediately.
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