Navigant Research Blog

An Electric Vehicle That Doesn’t Need a Diet

Dave Hurst — September 22, 2010

Like many reality TV stars, most electric vehicles (EVs) seem to be looking for ways to shed weight. As a result, manufacturers are looking to new lightweight, compact chemistries like lithium ion to reduce battery weight. These lightweight batteries help reduce overall vehicle weight and take up less space in the vehicle where occupant and cargo space is at a premium.

Interestingly, though, there is one electric vehicle that actually can’t use lightweight batteries without adding extra weight, a locomotive. Locomotives need friction between the steel wheels and the track in order to be able to pull the weight of the train cars from a stop or up an incline. This is called adhesion, and it’s a relatively simple set of calculations:

Adhesion = Coefficient of Rail Friction x Locomotive Adhesion Variable

Tractive effort = Locomotive Weight/Number of Axles x Adhesion

I should clarify that while these calculations are simple, the inputs are very complicated and the calculations change depending on whether starting from a standstill or already running (there are entire academic papers discussing the calculation of tractive effort).

Without getting too bogged down in the details, the locomotive adhesion variable is a measure of locomotive’s friction at the wheel touch point, and includes a wide range of variables: wheel conditions, bearing resistance, electric motor efficiency, and more. The coefficient of rail friction, according to Newton’s laws of motion, is the weight times gravity times a coefficient of friction, which will change with rail conditions (icy, oily, clean and dry, etc.).

All of this is a long way to get to the point that locomotives need weight for better adhesion because the friction will be higher. Norfolk Southern last year showed a prototype for a battery electric locomotive that utilized lead acid batteries for the storage. Based on my back-of-the-envelope calculations, I estimate the batteries to weigh roughly 16,000 lbs., and they actually help increase the tractive effort with their weight. If lithium ion had been used, the battery pack would likely weigh between 900 and 2,600 lbs. This results in a loss of the total tractive effort of the locomotive. A rough example might be:

This difference equals about 6% of the total tractive effort of the locomotive. The tractive effort helps determine the amount of horse power needed, which increases if the tractive effort is lower. So, unlike automobiles that are looking to shed weight, the weight of lead acid (or better yet advanced lead acid) is appealing to battery locomotives. Well, the weight and the much lower cost is appealing too, of course.

One Response to “An Electric Vehicle That Doesn’t Need a Diet”

  1. Matt Stevens says:

    Great article. Another category of electric vehicle that enjoys tipping the scale is the lift truck category (forklifts). For those vehicles it is not adhesion, but rather counterbalance that is needed. Extra energy capacity would help, but weight savings do not.

    It’s also worth noting that new sales in this category are 60% electric (electric rider and motorized hand). And that there are local leaders in this EV industry, such as Ohio-based Crown Equipment.

    Again, great article. Look forward to reading more of the posts from the Pike team.

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