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Semi (and other) fuel efficiency

Discussion in 'Semi' started by adiggs, Nov 4, 2019.

  1. adiggs

    adiggs Active Member

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    Created here out of a thread that gained life in the Investor forum.

    We got to talking about train vs. semi efficiency (diesel and electric), and then platoon / convoy efficiency.


    So in my vocabulary, trucks in platoon and trucks in convoy are the same thing (namely - incremental trucks after the first that are using software and driver assist technologies to follow a lead truck closely). Whether the 2nd / 3rd / etc.. trucks also have a driver on board is related but different - I can see early implementations where the later trucks have a driver on board, but those drivers can nap / watch TV / take a break, and can periodically rotate lead and follow positions.

    Later, the follow trucks can be driverless. For the current conversation, this is about the efficiency of the vehicles, and that doesn't change with or without the driver.
     
  2. adiggs

    adiggs Active Member

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    Yeah - that's what I'm thinking too. I figure the really long and big bulk shipments, train can hang onto that. But these really big, high capital cost industries need seriously big volumes to make them pay. (As we're seeing with the auto industry). Nick a little bit of volume off the top, and the business might stop being economical.

    (And I claim what would really being driving the lack of economics isn't the small incremental loss of volume - it's the unpriced externality commercial trucking gets, where road dmg from trucks is paid for mostly by society and only in a minor way by commercial trucking, vs trains (in the US) which pay for their own road surfaces).
     
  3. adiggs

    adiggs Active Member

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    @Rashomon said

    What I'm worried about - throw in hefty subsidization (roads) compared to trains, and we might even succeed in putting our big train networks out of business (oops).
     
  4. adiggs

    adiggs Active Member

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    This particular use case, to me, is a perfect example of where Tesla SHOULD get all of their freight moving on their own technology. They'd get direct feedback on their technology, on a controlled and increasingly well known route, and that will help them develop their semi technology for the wider market.

    It'll also be economically advantageous. And they might get platoon / convoy technology working between CA and NV to further improve the economics.

    And it's such a trivially small rounding error of the overall freight market that it's a perfect example of where we should be experimenting with alternative freight arrangements.

    And it's convenient for the 'line' to run direct from factory to factory.


    To extend the question and make it as close to an analogy with rail freight as possible, we'd make the answer to your question "free - the government will pay for the direct rail line and also pay for the maintenance". Then you can start assessing the operational costs between the two modes of transport.

    (But either way, it's not material to the overall freight market - even if it were slightly more expensive, Tesla should be running their own trucks between the two factories so they can get real truck fleet learning going on)
     
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  5. adiggs

    adiggs Active Member

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    My electric <> diesel efficiency conversion for class 8 semis works out to 2 kWh per mile electric vs. 6 mpg diesel. Tesla has said <2 kWh per mile and I'm using 2 kWh until we have more volume of data. Fleet average for diesel semi's has been 6 mpg from sources that I've looked up.

    https://afdc.energy.gov/fuels/fuel_comparison_chart.pdf
    This source provides us a comparison between gas and electricity (1 gallon of gas = 33.7 kWh electricity), with other fuels ranged out on that scale. Diesel weights in at 113% of the energy content of 1 gallon of gas - I make that 33.7 * 1.13 = 38 kWh of energy in 1 gallon of diesel.

    38 kWh of energy in 1 gallon of diesel moves you 6 miles - let's call that 6 kWh per mile worth of energy in a diesel semi. This accords with my own intuition that electric drivetrain converts roughly 90-100% of energy into motion, and fossil fuel drive trains convert maybe 30% of energy into motion (that's not a precise scientific statement - that's a cross check to confirm we're in the right ballpark with the previous calc).

    So electric miles are more efficient by a factor of 3 or so (these are all round number to establish order of magnitude or maybe even factors - not intended to be exactly precise).

    So I'm not missing the point - yes, semi's in platoon will be more efficient than semi's not in platoon. Diesel and electric semi's, at least as a physics principle, have equal access to platooning (two trucks can drive close together). The software that does that might not have as quick of a vehicle response for diesel vs. electric, but it's not an inaccessible proposition. Of course, if it's only been written and available on Tesla's semis, then the efficiency gain is only available to Tesla using platoon on the highway.


    Let's keep clear about what's being measured and compared. The start of this topic was energy efficiency. The big order efficiency gain is the way electric motors convert energy into motion, vs the way that Diesel engines convert energy into motion. There is approximately 0 energy efficiency gain by removing a driver from a second truck (but heck - that second truck can carry an extra ~200 lbs load!) - it's also obviously highly beneficial to the economics of moving things via truck.

    More broadly, there are at least these important metrics that measure different important dynamics.
    energy / mile to move stuff
    fuel cost / mile to move stuff
    cost / mile to move stuff (as measured by driver cost + fuel cost)
    complete cost / mile to move stuff (amb driver, fuel, maintenance, etc..)

    And realize that when we're comparing cost/mile to move stuff via truck compared to train, the trains (at least in the US) are operating in an environment where they pay their fuel, driver, and vehicle maintenance themselves IN ADDITION TO their infrastructure build and maintenance costs.

    Truck companies and trucking pay a small fraction (I haven't found a source that will put a number to it) of their infrastructure build and maintenance costs. The study I did find previously modeled the cost of building and maintaining the road infrastructure as a function of two variables - weather and heavy truck loads (they might have modeled it in terms of axles > XX weight, so an individual truck would count as 5-10 axles).

    The point is that all of the personal vehicle use of the road rounded to 0 for the maintenance of the road. That is the effect of road degradation being a function of the 4th power of axle weight.

    On the fourth power law, here's a readable blog post that illustrates the idea:
    The Fourth Power Rule

    And seems to be a pretty good summary of a more complete survey such as this:
    http://www.nvfnorden.org/lisalib/getfile.aspx?itemid=261

    Where we measure activity / load on a road in ESAL units (equivalent standard axle loads) where the reference heavy vehicle axle of 10 ton load has ESAL10=1 (you need to squint to get a reading from light duty vehicles).
     

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