Maximum MPG out of a diesel

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Apr 12, 2009
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In my experience, you can optimize efficiency in a diesel if you keep it in a certain RPM range as much as possible. That range differs slightly from engine to engine, but for many diesels the sweet spot is 1,900-2,100 RPM.
That is correct and great advice. I have very little experience with actually building diesel engines (only 'refreshed' them), but they do have a scientific sweet spot. Stoichiometry is involved in the calculations and the engineers gear accordingly, and staying in that RPM range makes a noticeable difference. In gasoline engines we would adjust the quench area.

Thinner oil fuel savings are not noticeable on an individual basis, it only makes an impactful difference on a global scale (many vehicles switching over).
 
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Oct 23, 2022
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Disabling the EGR often improves efficiency. But not always. Some engines have injection timing that is altered to accommodate the high EGR flow. One thing is clear, today's best "on road" modern diesel engines with cooled EGR systems and DPF are nearly all pushing 44% thermally efficient. That is exactly the number achieved before all this "tech" in the late 1990's.
Definitely true on the old Geo Metros - deleting EGR messes with the map and delays warm-up. My 94 gets 52mpg at 70 mph
 
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Sep 20, 2021
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Thinner oil fuel savings are not noticeable on an individual basis, it only makes an impactful difference on a global scale (many vehicles switching over).
I see no difference running 0W-20 or 5W-30 in my LM2. I've had both when running long distance trips (700+ miles one way at highway speeds). I prefer the higher shear of the heavier oil in the summer when towing so switch to the 5W-30 in the spring and back to the 0W-20 in the fall. YMMV
 
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Jan 4, 2023
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If you can find it, take a look at the efficiency graph of your engine and it's BSFC. Maybe this can help you determine the proper speeds and loads.

Disabling the EGR often improves efficiency. But not always. Some engines have injection timing that is altered to accommodate the high EGR flow. One thing is clear, today's best "on road" modern diesel engines with cooled EGR systems and DPF are nearly all pushing 44% thermally efficient. That is exactly the number achieved before all this "tech" in the late 1990's.


Good turbocharging, proper injection timing, and unrestrictive intake and exhaust are all important factors in modern diesel engines.

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dpf doesn't raise efficiency afaik, and scr is just magic sauce in the exhaust pipe for emissions. a dpf does increase backpressure which might get better economy. the main reason modern diesels get better efficiency is they run higher boost, the injectors spray finer mist, and the computer limits excess fuel. (if you floor an old mechanical diesel, you get all the fuel while if you floor a modern diesel you only get the fuel you need)



also short tripping a diesel (like only 10 km) wont let it warm up fully and you might have carbon buildup problems (more common on egr/dpf diesels)
 
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down in the park
Volvo make a 'Methane Diesel' engine which in light-load situations uses gas as the main fuel and diesel as the pilot. Where extra torque is needed, the gas it replaced with diesel. Quite interesting.

I worked asa mechanic at a bus company (I'm a driver now, for a different company) where they converted busses to run on LPG before my time. Spoke to the old engineeer who did it, apparently they could run on lpg from cold but smoked a lot until warmed up.
 
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Mar 10, 2017
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I worked asa mechanic at a bus company (I'm a driver now, for a different company) where they converted busses to run on LPG before my time. Spoke to the old engineeer who did it, apparently they could run on lpg from cold but smoked a lot until warmed up.

I'm assuming these were petrol powered busses initially?
 
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