Seasonal affect on diesel engine fuel economy

[vgs1133]

Hello,
I am new to this forum today so if I am not posting this question in the proper place please correct me.

I am trying to factor out seasonal affects on fuel economy in a fleet if municipal buses so that I can better evaluate certain proposed fuel saving measures. Does anyone have data on this? The fleet is in North Carolina. The engines are primarily Cummins. The fuel is LSD.

 

[waross]

I would call the local fuel supplier and determine whether the specific gravity or other charachteristics of the fuel are changed seasonally. I understand that there can be as much as 10% difference in sg. of fuel between winter and summer in colder climates. As you know the heat content of the fuel is related to the sg.
respectfully

 

[patprimmer]

Yes

The heat value is related to mass and the fuel is actually metered by volume, hence SG is critical.

Also, warm up times and manner of use might change due to weather. Passenger travel patterns might also change, so there will be factors other than MPG that impact on the amount of fuel used.


Regards

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[Rob45]

Pat;
Depending on the age of the engines, you could be wrong: current Cat engines (I know, the buses have Cummins, but I know about the Cats) try to meter by mass by inferring the spec grav. from the fuel temperature.
Thus, the engine will burn a larger volume of fuel in hot weather than in cold weather, but the guy measuring the "fuel economy" will be measuring only gallons, and so will show worse fuel economy for hot fuel than for cold.

VGS: You certainly need to compensate for temperature when measuring fuel economy, or your numbers will be at best misleading. You should also regularly sample the fuel yourself for spec grav., since your supplier probably won't know.
Finally, the grade of fuel is important, whether it's number 1 or number 2; these typically have very different cetane numbers and could well have other differences that affect fuel mileage.

 

[patprimmer]

I could well be wrong, as I was thinking older mechanical pumps.

Hotter weather means less dense fuel, but also generally less dense air. The air density has an effect on both A:F and aero drag.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[vgs1133]

I would like to thank you all for your comments. I have been told by a number of people, including folks with the EPA and at Oak Ridge Nat Lab, that fuel economy drops in cold weather for a number of reasons, some of which have been identified here. However, the Scandia company published a graph a few years ago showing a clear correlation between season and economy that is 180 deg contrary to the common belief (span ~9%). I am looking at 2 years of data for the Cummins engine that seems shows the same thing in North Carolina (best in Dec-Jan worst in Jul-Aug). Cummins responded to an inquiry with the explanation that MPG gets worse in cold weather. I also have data on a group of buses with DD50 engines (all are post '96) that does not show such a clear trend. We are monitoring the performance of a modification we made that appears to improve economy by ~9%, but it is difficult to get the change to stand out if the overall is declining. I was hoping to find other studies that might help.

Thanks, and keep the ideas coming if any new ones come to mind.

 

[Rob45]

When we do fuel economy testing on trucks, we start with (at least) two identical trucks and first establish a correlation between them, over the period of a number of runs, and learn the average variance between the two.

Then we make a modification to one of them, and run another series of comparison runs, both the modified and the baseline vehicles.
We run both trucks on the same route, same time, same day.
Not close together, of course, but within a minute or two of each other, so that both experience the same traffic, wind, temperature, etc. conditions.

The fueling process itself is almost the subject of another full discussion, but suffice it to say we do our best to compensate for variations in fuel temperatures and for the variation in apparent fuel level during fueling.
We're careful always park the truck in exactly the same place when fueling, and to only fuel one tank, in order to avoid trying to compensate for fuel transfer between tanks.

We have found, incidentally, large variations between the fuel mileage number obtained by our measurements, and the fuel mileage number computed by the engine control computer.
The ECM infers fuel usage from injector rate and pulse-width rather than actually measuring fuel flow, and tries to compensate for volumetric changes using fuel temp, but Caterpillar at least have a defective algorithm, so their result varies wildly, and is effectively useless.

 

[drwebb]

Just to throw an idea out- if ridership declines in winter because people don't want to stand outside waiting for the bus, then they might blow past several empty stops changing their duty cycle.

For example, when I was at UNC several routes had to add extra buses when it rained and folks didn't ride their bikes or walk to campus, so I'll bet they got worse mileage when it rained- but was it from the wet?

 

[Rob45]

You will only get usable fuel consumption information iff you run a standardized route with constant load, and with a comparison vehicle.

Unless of course you're willing to wait months or years to accumulate enough data for daily variations in ridership etc. to average themselves out.

 

[vgs1133]

Actually I have two years of data for a group of 43 buses to work with. The data is not the cleanest (I have been provided with monthly averages and I believe that days with no travel were indicated by "0"s, and cells with "0"s were then included in the monthly averages) so we are using a statistical approach to reveal long term (seasonal) cycles. I does appear that an annual cycle exists that is contrary to the common belief that economy will decrease in winter. The monthly data also shows that the expected peak in December was not realized, and this may be due to a change in use pattern, the holiday shoppers, which is in line with drwebb's suggestion. Incidentally, drwebb, the Chapel Hill transit company is one of the groups we have been talking to.

If anyone knows of someone who might like to get involved in a more direct test like Rob45 suggests (a manufacturer, fleet manager, or end user) I'd like to talk to them. If this request violates forum policy, please delete it.

 

[rmw]

Other factors enter into fuel mileage than just fuel conditions or consistency.

Lube oils in axles and hubs has a higher viscosity and hence more drag. Same in the trans. Some fleets are slow to adjust tire air pressures to compensate for seasonal changes, so tires can be softer.

Lots of variations enter into the picture.

rmw

 

[Rob45]

VGS:
Why don't you contact the bus mfr and the engine mfr, see if you can get them interested in a joint project?

They might be interested since fuel economy seems once again to have gotten important, at least for a while.