Getting Better Fuel Mileage

I've spent a lot of time with trucks, truck engines and fuel mileage. With high fuel prices and bogus devices on the market. I decide to write up an article.

(This is the truth, but it might not be what everyone wants to hear)

The #1 factor in getting better fuel mileage is to REDUCE HORSEPOWER DEMAND. This is a combination of vehicle configuration, condition, load and DRIVER OPERATION.

The #2 factor in getting better fuel mileage is to AVOID RUNNING THE ENGINE WHEN YOU ARE NOT MOVING.


The #3 factor in getting better fuel mileage is the engine.
Everyone wants to look at the engine first , because they think the part that burns the fuel is the most responsible for fuel economy. I'll discuss this first and then get to the other issues.

Most modern on highway diesel are capable of BSFC's in the low .300's or better (high .200's) at cruise rpm. This is pretty #### lean compared to earlier engines and I haven't seen any after-market accessories offer an improvement.

So, as far as the engine, you just need to make sure the engine is mechanically/electronically correct to start.

Check air and fuel filters.

Inspect air intake and exhaust systems. Look for leaks in the after-cooler piping, after-cooler core etc... Best way is to plug at each end of after cooler piping and pressurize entire assy to test for leaks.

Check exhaust for blown manifold and turbo gaskets. look at manifold and turbo hot housing for signs of overheating. A visual of the cold side is helpful, looking for damaged fins, rubbing etc...

If mechanical engine, check timing. Also check linkage to pump. Check fuel settings if seals or broken, Check air air fuel ratio controls, (aneroid, puff limiter etc..)

If electronic, check for fault codes and possibly do a timing and sensor calibration.

Check fuel pressure.

Make sure engine is up to operating temp. Usually 190°

Check fuel quality: Fuel should be 35 API at 60°. Thinner winter blend will reduce fuel mileage considerably.

Fuel heaters: should be thermostatically controlled. Off in summer and only on when needed in the winter.

Oil: use 10w30 in all but the hottest weather. If you are running 15w40 in the winter it's hurting fuel mileage. You will see 4-5psi lower oil pressure, but it won't hurt anything.

Clutch fan: must cycle on and off. Fans require 15-18hp to run.

BLACK SMOKE: There should be no black smoke when changing gears or accelerating. Black smoke means the engine / fuel system is out of tune and adjustment for fuel economy. BLACK SMOKE is MONEY going out the exhaust pipe.

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VEHICLE: many considerations here. Start with tires. make sure they are properly inflated (wish i had $$$ for every tractor that showed up for a dyno that had low tire pressure)

Tread design: rib is better than on/off road and heavy lug tread is terrible, run rib tires when possible. Low profile has much lower rolling resistance too.

Drive line: check drive-shafts for misalignments and vibrations. Rears must track straight, worn misaligned rear suspensions can cost you. An eyeball along the edge of the rears to the front can give you a quick check here. Use fuel saving synthetics in trans and differentials.

Vehicle aerodynamics / configuration: This varies greatly with different type of power units and trailers. The best possible is a Cab-over-engine (COE) with roof fairing, side fairings and very few protruding accessories, pulling a smooth side van trailer. Everything having ribbed low pro tires

The worst is a big conventional with no fairings, big external air cleaners , big mirrors, horns, fuel tanks, visors, no fairing, pulling a flatbed with a big bulky load, with lugged tall tires.

HORSEPOWER DEMAND: The horsepower that is needed to maintain a certain speed. i.e. how much horse power needed to overcome wind and rolling resistance, determines the fuel you will burn to cruise down the highway. This is called HORSEPOWER DEMAND. It also determines how much RESERVE HORSEPOWER is available for pulling hills and passing other trucks etc...

EXAMPLE: If two trucks have 400hp and truck #1 requires 160hp to maintain 60 mph and truck #2 requires 220 hp to maintain 60 mph, truck #1 will of course have better fuel economy AND have 60 more RESERVE HORSEPOWER to pull hills.

So aerodynamics play a huge role in fuel economy, particularly at speeds above 55MPH
(55 wasn't a arbitrary number when it was the federal speed limit)

Above 55MPH "AERODYNAMIC HORSEPOWER DEMAND INCREASES BY THE CUBE OF SPEED INCREASE".
How this plays out is a truck that can achieve 7 MPG at 55 MPH, will get 6 MPG at 65 MPH and only 5MPG at 70MPH

Aerodynamic considerations include having a roof fairing that matches the height of the trailer. The trailer should be as close to the tractor as possible. Ideal distance is 15-18 inches, never more than 24 inches. Side fairings help close this gap without the trailer being to close for sharp turns.
At distances over 24" inches between the trailer and the tractor, drag goes up considerable. by 48" you have the drag of TWO FULL FRONTAL AREAS. Might as well have the trailer beside the truck! So much for the stretched out "west coast" look.

It can be tough to improve an existing tractors aerodynamics, but you can add a roof fairing and side fairings. Avoid putting on a lot of bulky accessories like visors, bigger mirrors, dual external air cleaner, huge bumpers, horns, flaps hanging everywhere, etc... Remember, driving an un-aerodynamic rig fast, will hurt fuel mileage even more.

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DRIVER OPERATION: This is the big one that no one wants to face up to. However there is a BRAND NEW PICK-UP TRUCKS WORTH OF FUEL SAVING$ PER YEAR AVAILABLE (to those that operate well VS those that don't)

#1 Cruise speed: I just went over aerodynamics and horsepower demand.
More time at lower speed will save you big on fuel.

#2 Cruise RPM: most modern trucks will operate efficiently in the 1400-1600 rpm range depending on model of engine. Cruise at this rpm (in the lower range of it if possible) Do not gear down a step to run at 1800rpm or higher

USE CRUISE CONTROL, many electronic engines alter timing maps in cruise control for better steady state economy

3# PULLING HILLS: Diesel have a fuel curve inverse to the torque curve. (on some electronic engines this flattens out)

A typical 400 hp engine may burn 18 gallons per hour (GPH) at 1800 RPM, But only 14 GPH at 1200 RPM. Meanwhile it has MUCH MORE TORQUE AT 1200 rpm.

What does this mean? You generally pull hills at max fuel. You want to pull hills in the highest gear you can and keep the engine at peak torque/ lower GPH.

A big mistake is to drop a gear and run at the hill at higher rpm where the engine uses more fuel and has less torque. This is a fuel wasting downward spiral were you have to make successive gear changes burning more fuel and going slower and slower.

Stay in the highest gear as long as possible in hills, if you can crest the hill at or above 1000rpm, it's usually o.k. to do so.
Think about the savings in hilly terrain, hill after hill BURNING 14GPH vs 18GPH to get the job done. Add that you are usually pulling at a higher speed with correct method so you spend less time at max fuel consumption.

#4 GEAR CHANGES: Shift progressively, use only enough RPM to get into the next gear. Taching out every gear is a waste of fuel, especially in city driving

#5 COOL DOWN: Use exit ramps and truck stop access roads to cool your engine down before shutting it off. If you can coast in to a stop using very little fuel and keeping the exhaust temp down, there is no need to let the truck idle several minutes before shutting it off. Idling is a big waste if fuel and five minutes has a way of turning into twenty five.

#6 IDLE TIME : THE BIGGEST WASTE OF FUEL OF ALL. Any time that engine is running and the truck isn't moving hurts tank mileage and your wallet. It is also unnecessary engine wear. Shut the truck off whenever you can. Cool engine down as outlined above. Consider alternate heat/cooling options for over the road trucks. These are best installed when the truck is new for maximum payback.
An electric blanket or good sleeping bag can get you a few more comfortable hours in sleeper without having the truck running.

#7 DRIVE SLOWER (did I say that yet?)

 

I don't know if this is sarcasm, but I'll respond.

The answer is this:

It would really be hurting fuel mileage if you are pulling a tall van trailer. Not so much if your pulling a low flatbed.

4ft and over gap is like pushing 2 truck fronts through the wind. Twice the frontal area.

The fewer "different" surfaces on a truck, the better. Although they are almost extinct, COE is the best because the smooth uninterrupted front actually creates an "aerodynamic bubble" in front and the air then slips past the smooth sides.

There were some very good COE's developed in the mid 80's through early 90's, starting with the GMC Astro "Bandit". This truck, while not a great unit from a drivers point of view, had a lot of good aerodynamic research behind it.

In the early 90's Caterpillar had some test trucks with 11 liter engines. They were COE tractor/ smooth van trailer with all the areo tricks, super single tires, side skirts, belly pans, smooth covered wheels.

they were driven with proper technique for mileage, averaging 11 mpg on a 900 mile round trip of miked terrain and elevations, 80,000 gvw, 54 MPH average speed. (slightly rounded figures)

 

I never thought truckers were not intelligent. Most are business men that have to be on top of things to survive. I wouldn't have had a career without a trucking industry.

Many owner operators and drivers are skeptical because of all the BS they run into. I try to give folks respect and attempt to present a good logical case when explaining things.

 

For most, cruise control, when you can use it, will yield better mileage, esp on easier more level terrain.

Many engines enter a steady state mode after a couple of minutes.
This will go into a different timing map and be a little more efficient. You certainly can do this without cruise, but it's more difficult to maintain a very steady RPM.

Cruise is never perfect. Obviously you cannot use it all the time. There are different ways to program it such as "soft" or "hard" cruise. Soft cruise will back off sooner when cresting a hill, but be more sluggish going in to the hill.

One way is to program cruise "soft" with and then you can override with the pedal to "warm up" the turbo as you approach the grade.

Now I'm not saying that you cannot beat cruise with your foot. I think it's stressful to drive that way, strictly controlling the engine and speed. Most drivers won't do it.