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sbphoto · 07-12-2011, 07:28 PM

Well, my last go-round seemed to stray into fuel efficiency territory. It caused me to think about something I don't know enough about but I am starting to find out what I can.

Given the usual size of a truck trailer or a toter/motorhome, a large percentage of fuel use for highway travel is because of wind resistance. The usual area being moved along the highway is about 110 to 115 square feet. I looked at several information sites (Kenworth has a good one) which show that at 65 MPH on level ground (yeah, yeah...there are hills and mountains), 2/3 of the power is consumed moving against air resistance. If one wants to save fuel, you can slow down...but we want to get to where we are going. Modifications in shape (aerodynamics) can modify power requirements and fuel use to some extent but here we are talking fairly small percentage changes.

The weight to power/fuel use is important starting from a stop, changing speeds, going up hills, and so forth. So, a heavier rig will burn more fuel going up hill than a lighter one will given the same size and shape and speed.

None of this looks at changes caused by using your generator in transit but my head already hurts so I am not going to think about that.

The fact I found most interesting and want to know more about is that for a given size and wieght truck/motorhome there is an optimal engine size. Going larger wastes fuel and does not pay off in much better performance (such as hill climbing) and going smaller also wastes fuel but does result in worse performance. There must be published tables about how to estimate this. It seems to me that many people decide that the easy way to deal with this is to get an engine with gobs of power (500 HP +) and enough torque to pull down a small mountain so they don't have to worry about "not having enough power". That works for the power part but then they get 6 to 8 MPG at ~$4.00 per gallon.

Does anyone know of a place to estimate what the best engine size is for a given rig size and weight???

07-12-2011, 07:28 PM	#14
sbphoto Junior Member Join Date: Jun 2011 Posts: 11	making my head hurt Well, my last go-round seemed to stray into fuel efficiency territory. It caused me to think about something I don't know enough about but I am starting to find out what I can. Given the usual size of a truck trailer or a toter/motorhome, a large percentage of fuel use for highway travel is because of wind resistance. The usual area being moved along the highway is about 110 to 115 square feet. I looked at several information sites (Kenworth has a good one) which show that at 65 MPH on level ground (yeah, yeah...there are hills and mountains), 2/3 of the power is consumed moving against air resistance. If one wants to save fuel, you can slow down...but we want to get to where we are going. Modifications in shape (aerodynamics) can modify power requirements and fuel use to some extent but here we are talking fairly small percentage changes. The weight to power/fuel use is important starting from a stop, changing speeds, going up hills, and so forth. So, a heavier rig will burn more fuel going up hill than a lighter one will given the same size and shape and speed. None of this looks at changes caused by using your generator in transit but my head already hurts so I am not going to think about that. The fact I found most interesting and want to know more about is that for a given size and wieght truck/motorhome there is an optimal engine size. Going larger wastes fuel and does not pay off in much better performance (such as hill climbing) and going smaller also wastes fuel but does result in worse performance. There must be published tables about how to estimate this. It seems to me that many people decide that the easy way to deal with this is to get an engine with gobs of power (500 HP +) and enough torque to pull down a small mountain so they don't have to worry about "not having enough power". That works for the power part but then they get 6 to 8 MPG at ~$4.00 per gallon. Does anyone know of a place to estimate what the best engine size is for a given rig size and weight???