Other Stuff

The difference between efficiency and economy, and how to evaluate fantastical MPG claims
A simple test to get a better number than guessing
The Problem of EV Sizing
Building a calculator to predict weight and battery capacity
Is a 100-mpg Gas Car Possible?
The holy grail of the automotive industry
EPA Fuel Economy as a Function of Vehicle Parameters
Fuel efficiency charted by weight for MY2025 alternative-fuel cars
Light Vehicle Efficiency Over Time
Tracking the relationship between resistance force and engine efficiency
How to Road Trip Efficiently
Planning, packing, and driving for least fuel consumption

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Testing a Smooth Engine Undertray

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When I investigated the effects of a splitter on my Prius , I discovered something unusual: gauge pressures on the stock engine undertray were a lot higher than I expected. Julian Edgar’s Vehicle Aerodynamics: Testing, Modification & Development includes several examples of engine undertrays with measured pressures much less than atmospheric. My test showed that the Prius undertray was developing pressures at atmospheric or higher. What was going on? Gauge pressure at 80 kph. Left: no splitter. Center: with splitter. Right: difference. Hypothesis and Testing   So, I’ve got a problem here I want to investigate: high pressures on the engine undertray where most examples I’ve read about have lower pressures. Where to begin?   When you investigate something like this, a good place to start is fundamental principles. I know that velocity and pressure are related, and that as pressure goes down, velocity goes up. So the velocity under my car’s engine undertray must be slower...
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How to Estimate Your Car's Coefficient of Rolling Resistance

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When two objects in contact with each other slide, a friction force develops resisting the motion. In classical physics, we distinguish between static friction (which resists the onset of motion as some force is applied) and kinetic friction (which resists movement after it has begun). You’ve noticed this before: the last time you tried to push a heavy box across the floor, say, you had to give it a push to get started and overcome the static friction (and if the static friction was great enough, it perhaps tipped over instead of sliding). When you stopped pushing, it didn’t continue sliding indefinitely; it gradually slowed and required some force to keep moving (although not as much as it took to get started). In both scenarios, your pushing on the box was required to overcome its static or kinetic friction. Upright, the fluid-filled can slides when pushed but as energy is dissipated due to friction with the table surface, it quickly comes to a stop. Cars are a bit more complicated b...
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How to Think About Car Aerodynamics: A Very, Very Basic Overview

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The study of aerodynamics is complicated. If anyone tries to tell you otherwise, run the other direction—it’s a sure sign they don’t know what they’re talking about.   Over the last two years especially, my thinking about aerodynamics and appreciation of its complexity has changed dramatically—a result of my going back to school to get another bachelor’s degree, this time in aerospace engineering where a good working knowledge of airflows is required and education in not just general fluid mechanics but also aircraft aerodynamic design forms a core part of the technical curriculum. I'm in the midst of my last semester now and to clarify my thinking at this point I decided to put some things in writing in the hopes they might help someone else as well as myself, specifically focused on car aerodynamics. A word of warning: I've tried to minimize the amount of math below, but some mathematical relations are unavoidable if you want to build an understanding of fluid flows. If anyth...
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