It is typical today that the terms Hypermiling and EcoDriving are synonymous. I am here to say that this is not so and revel information about vehicle emissions that no one wants to talk about.

I will define EcoDriving here as the act of driving to minimize vehicle emissions. It is my belief that the reduction in vehicle emissions is more important than maximizing fuel economy. After all the hybrid electric vehicle was created not to reduce fuel consumption but to reduce harmful vehicle emissions.

In many ways, the reduction in fuel consumption and the reduction in vehicle emissions can be achieved using the same techniques. There is, however, one Hypermiling technique that is counter-productive to minimizing vehicle emissions. But first, some background information.

Vehicle Emissions and Acceleration

The news media always refers to vehicle emissions as a fixed amount. So many milligrams of hydrocarbons (HC), carbon monoxide (CO) or oxides of nitrogen (NOx) per mile or kilometer. The reality is that vehicle emissions are not fixed, they vary a lot by vehicle, whether the vehicle is cold or warm, under load (and how much that load is) and especially by vehicle acceleration.

Studies have shown that (assuming that the vehicle is warmed up):

• emissions are lowest at idle
• the best type of driving for low emissons is a slow constant speed
• any acceleration dramatically increases emissions
• the amount of emissions goes up exponentially not linerally the more you accelerate
• the lower the speed, the lower the emissions

Based on the above information, the best way to accelerate (if you have to) is as slow as possible (without being a safety hazard).

Hypermiling's Approach to Vehicle Emissions

Hypermiling advocates a technique called pulse and glide (or P&G for short) where you would accelerate to the desired cruise speed and then glide back down to a minimum speed. In other words, not maintaining a steady speed.

Why I Don't P&G

Since vehicle emissions are lowest (assuming the vehicle is warm) at a steady speed (except for idle), P&G is counter-productive to minimizing vehicle emissions.

Some advocates of P&G claim that the engine is most efficient at 75% of load or roughly 3/4 of full acceleration and advise 3/4 acceleration during the pulse phase. This is deadly behavior for minimizing vehicle emissions. We have already learned that any acceleration increases vehicle emissions and the amount of emissions increase exponentially as acceleration increases.

References:

• Estimating Vehicle Fuel Consumption and Emissions Based on Instantaneous Speed and Acceleration Levels Kyoungho Ahn, Hesham Rakha, Antonio Trani and Michel Van Aerde
  
• Positive Mean Acceleration for the Determination of Traffic Emissions Mauricio Osses, Alvaro Henriquez, Rubén Trivino