What’s All This Regenerative Braking Stuff?

The late Bob Pease came up in a recent blog response so I decided it was time to pay a tribute to him. Bob used to write a column with a title of “What’s all this (insert subject here) stuff?” My thoughts about this column ironically came to me when I was hiking in honor of another Bob on the 4th of July. As a landscape oil “wannabe” artist, one of my mentors was Bob Ross who passed away on July 4th, 1995. In honor of the twenty year anniversary of his passing, I took a hike on a Hogsback Ridge in Colorado. The ridge was right at the bottom of the place where Interstate 70 has a long, eastbound descent from the Rocky Mountains onto the Great Plains.

This slope not only wreaks havoc on semis as they climb into the Rockies, it also causes many problems for the trucks that are descending the rapid elevation change. There are a series of bright yellow warning signs for truckers to be in the proper gear before descending. There is also a Runaway Truck Ramp for those that don’t gear down. When that happens, the trucks are left to their brakes as the only form of stopping. Often times the brakes overheat and become ineffective. The only way to stop then is to use the Runaway Truck Ramp which has a long lane of gravel that provides resistance for stopping a truck. At the end of the ramp are some crash absorbing barrels to take away the final momentum.

My college physics professor always emphasized the fact that energy was conserved. As I stood on the ridge, I could smell the energy….or at least smell the residuals of it in the form of hot asbestos. I thought of how capturing the energy was a much more efficient way of braking. I thought about regenerative braking.

Regenerative braking is not a new concept. It has been around for a long time. Locomotives have been using it for years. Just recently, it has found its way into medium duty trucks, buses, and cars. There are two main methods for capturing the energy. One is to brake using an opposing electrical force and the other is to capture energy as hydraulic pressure. The energy is then returned to the drivetrain when the vehicle accelerates using the same mechanism that captured it. Electric systems store the energy as charge whereas hydraulic systems store the energy as pressurized fluid in a reservoir known as an accumulator.

As electrical systems advance, they are taking more and more market from mechanical solutions. I decided to make a table of comparison to see which regenerative braking technology was more efficient. As a power electronics engineer, efficiency is always a goal of mine. I wanted to capture the efficiency increase of each system based on the application. My research did not reveal a direct comparison however the tables in Reference [3] do. Instead of efficiency, I recorded the claims made for increases in MPG.

Table 1

Regenerative Braking Applications, Technologies, and Claimed MPG Increases

Regenerative Braking Applications, Technologies, and Claimed MPG Increases

The hydraulic systems have a slight edge in medium duty truck applications at a comparison of about 25-27% electric to 30-35% MPG. For a sedan, the hydraulic solution raises MPG by 52%. Although I couldn’t find any percentage numbers for electric systems, I did find that reference [8] states the numbers are often inflated; “Thus the EPA admits that even after its most recent revision of fuel economy test procedures, which took place in 2008, the ratings for hybrids can easily be 20-30 percent higher than most drivers will experience in real-world conditions.”

In addition to efficiency and MPG, cost, weight, number of cycles, and safety were some of the main functions for sales related issues. Batteries are the undesired for the low number of cycles and safety hazards as well as the added weight. Capacitors offer some advantage in these areas [1]. Whereas the hydraulic regenerative systems were heavier, the storage systems weighed less than their electrical counterparts (without the oil added). As a power engineer, the interesting aspect of the electrical systems is that they require an inverter. In addition to power electronics, some systems offer an interactive display and control module that monitors the performance.

As I close this blog, I wonder if Bob Pease had ever considered capturing the energy in his old VW bug using regenerative braking. It sounds like something he would have attempted. I wonder if Bob Ross would have captured the beauty of Red Rocks park that was below me on the ridge. It sounds like something he would have attempted. So long Bobs. Thanks for making the world a better place


  1. “The Kinetics Hybrid Delivers Proven Cost Savings and Emissions for America’s School Bus Fleets” case study report
  2. Allison Transmission/Hybrid
  3. “Digital Displacement Hydraulic Hybrids, Parallel Hybrid Drives for Commercial Vehicles” Taylor et al, JSAE National Annual Congress, May 24, 2011
  4. “Hydraulic Regenerative Braking Saving 52.7% Fuel in BMW 530i” Ovidiu Sandru February 26, 2009 Hybrid vehicles
  5. “Eaton Hydraulic Launch Assist”
  6. “Eaton Hydraulic Launch Assist Refuse Truck”
  7. “Lightning Hybrids Sales Sheet”
  8. “The Real Costs of Owning a Hybrid, Do Fuel Savings Offset a Higher Price?” Republished: 09/09/2013 (Original Date: 11/16/2004)

2 comments on “What’s All This Regenerative Braking Stuff?

  1. ssco00
    July 16, 2015

    The Toyota Prius does this, charging a battery that can drive the vehicle.  Going downhill in curiuse control, it automatically goes into that mode to hold speed close to the set value and charge the battery in the process.  I make the last part of my trip home on slow resedential streets using battery power alone.

  2. Effective-Technical-Writing dot com
    July 21, 2015

    to scco00,

    Thank you for your response to my blog.  The ability for cruise control to slow a vehicle as well as accelerate it is surely a welcome addition to vehicle intelligence.  I am aware of several “speed traps” where radar is waiting at the bottom of a hill.  Cars will naturally accelerate when going downhill.  I recently rented a Ford Escape that actually downshifted the transmission to slow the vehicle.  Of course capturing the energy in regenerative braking is an even better feature.  The main point is, you can avoid a speeding ticket due to this innovation.  So the cost savings goes beyond energy reduction.  What's not to like about that?

    As for using only electric power in your neighborhood, that is the benefit realized when capturing energy during regenerative braking.  Again, energy is always conserved.  It can be thrown away as heat or used to power your vehicle.  Technology enables the latter.  


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