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Assuming we have 2 identical cars. What is the total energy consumed by the two cars to start and accelerate up to a specific speed versus the total energy for one of them to accelerate up to a specific speed having the other tied to it? Or the fuel consumed for two vehicles to run on a road at 50mph and the fuel consumed for one vehicle tied to the other. I basically don't want an exact Joules number or Fuel litres. I need to know the difference between each of the cases. Which is most economical and by how much?

sloupioc
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    There are far too many unknown variables in this problem to provide any kind of answer. Consider editing it to make it more specific. – grfrazee Jul 29 '15 at 16:46
  • what else could I write? I basically don't want an exact Joules number or Fuel litres. I need to know the difference between each of the cases. Which is most economical and by how much? – sloupioc Jul 29 '15 at 17:02
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    Have a look at the Help Center. See here as well for some help as to what constitutes a good question. SE is more set up to answer specific, limited questions, not to answer general ponderings about a subject. – grfrazee Jul 29 '15 at 17:33
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    Also, if you hover your mouse over the "Thumbs Down" button, you will see that it says "This question does not show any research effort; it is unclear or not useful." Your question shows no research into your topic to give anyone a starting point, and your input parameters are unclear, or do not given enough input to begin solving the problem. (As an aside, you're mixing SI and Imperial units, which is generally a no-no). – grfrazee Jul 29 '15 at 17:38
  • Related: Does having 2 engines together increase anything? – Air Jul 29 '15 at 19:57
  • @sloupioc, your questions are too broad. All of the members of this community are volunteering there personal time. In order to get help it is best that you do some research and share your research in the body of the question. I noticed that you already have a question on hold. I am voting to close this question. – Mahendra Gunawardena Jul 30 '15 at 11:25
  • @Mahendra Gunawardena you are extremely wrong. I discussed this with a senior automotive engineer and he said it's a brilliant question, but not straight-forward to answer. He gave me some insight. The question is very specific, contrary to what you are saying. Maybe instead of voting the closing of a brilliant question, you should elaborate what is the problem with this question. All factors are considered the same, same engines, same acceleration, same everything. So there is nothing broad about this question. Please refrain from closing people's questions because you simply cannot answer. – sloupioc Jul 30 '15 at 12:24
  • @sloupioc, I would agree this is brilliant question may be for a Phd thesis or a research project, but in the context of this forum, and considering that the respondents on this forum volunteering their personal time to respond, I would have to say the question needs improvement. BTW I spent over a decade in the automotive space, and I rarely vote to close questions. – Mahendra Gunawardena Jul 30 '15 at 12:34

1 Answers1

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In your scenarios, there are two places that the energy ends up: as kinetic energy in the vehicles themselves (0.5 × mass × velocity squared) and as losses (rolling resistance, air resistance, etc.).

There are two possibilities regarding your two scenarios.

  1. The two scenarios occur in the same amount of time. This means that the total energy is the same in the two scenarios, and that in the two-engine scenario, the engines are operating at less than half their capacity. In this case, the energy consumed would be based only on the relative efficiency of the engines at the two different power levels.

  2. The single-engine scenario takes more time to reach the final speed than the two-engine scenario. In this case, while the final kinetic energy is the same, you have more losses while getting there, and therefore more energy input will be required, regardless of the engine efficiency numbers.

Dave Tweed
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  • your analysis starts well but I am not sure about the conclusion. Indeed in both cases (single engine dragging two vehicles VS two engines dragging each vehicle) the kinetic energy acquired is the same. So we should focus on the losses. At the moment I really cannot say if in one or the other case there are more losses. I have a "feeling" that if one engine drags two vehicles will be more efficient than having two engines dragging each vehicle. More moving parts, more friction. I am not sure, it doesn't seem clear enough. Any other idea?? – sloupioc Jul 29 '15 at 18:50
  • I tend to agree with @David Tweed, one addition to Scenario 2 is for both cars to reach the same speed in the same time as 2 cars operating there engines independently (Scenario 1), the one engine powering both cars would need to work much harder (if it was capable of doing so) & use more fuel – Fred Jul 30 '15 at 00:59
  • yes, it would need to work harder, but usually engines are more efficient with increased torque/rpm. Also it is not the same to have a 2 ton car with having 1 ton car and another 1 ton car tied together. The rolling resistance increases, but not proportionally I think. – sloupioc Jul 30 '15 at 12:26
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    Where did the 2-ton car come from? Until now, we have been discussing the "2 identical cars" that you posited originally. – Dave Tweed Jul 30 '15 at 13:39