Engineering Hydraulic Hybrid Vehicles

Using STOP to power GO

A global clean-tech opportunity

An alternative to electric hybrid vehicles, hydraulic hybrids transfer kinetic energy generated during braking to an “accumulator” of pressurized fluid, which then provides energy when the vehicle accelerates.

The technology uses hydraulic hardware to augment the conventional powertrain (engine and transmission) of a vehicle, reducing fuel consumption—and therefore vehicle emissions—by recovering and then using energy that’s typically wasted.

Hydraulic hybrids recover energy more efficiently than electric hybrids and can improve fuel economy by 15-30% for parallel systems and by over 50% for series systems. The technology can also extend brake life 2-4 times in certain applications.

The best application of hydraulic hybrid technology is in heavier vehicles with high dynamic-driving-cycle operations, typically commercial vehicles that make frequent stops: more braking, more energy capture.

Targeting an untapped market

We know that existent, proven hydraulic-hybrid regenerative-braking technologies can significantly increase fuel economy, reduce particulate and greenhouse emissions, and reduce maintenance costs for larger vehicles with high dynamic-driving-cycle operations. The technology has been around for decades, and there are millions of vehicles on the road that are suited for hydraulic hybrid use.

What’s stopping us from cutting fuel use and emissions in half for millions of vehicles?

Currently, most hydraulic hybrid R&D targets OEMs interested in implementing the technology in new vehicles. While valuable moving forward, these solutions don’t address the fuel consumption and emissions of millions of vehicles already on the road.

The feasibility of a new retrofit approach

Project 1: Assessing the feasibility of hydraulic hybrids for developing nations

The engineers at Czero have been developing hydraulic hybrid vehicles since the early 1990‘s, and in partnership with the Colorado State University (CSU) Engines & Energy Conversion Lab (EECL), we launched a study to assess the feasibility of developing a cost-effective, easy-install hydraulic hybrid technology for use in commercial vehicles.

One of our primary objectives was developing a solution viable in developing nations, where

  1. The cost of fuel relative to per-capita income is high, thus providing a strong financial incentive to adopt the technology;
  2. The opportunity for environmental impact is greatest because of the number of older, more polluting vehicles in use.

Feasibility study objectives

Vehicle retrofits can be complex and expensive, so to develop a commercially viable retrofit kit—especially one viable in developing nations—we established these project objectives:

  • Minimize installation costs
  • Utilize mostly off-the-shelf components
  • Minimize custom parts needed
  • Keep controls integration simple
  • Utilize existing manufacturing and distribution chains

Results of the hydraulic hybrid feasibility study

Continuing our collaboration with the CSU EECL—and supported by generous funding from the National Collegiate Inventors & Innovators Alliance (NCIIA; now VentureWell)—Czero quickly took the project from concept to prototype.

In twelve months, on a shoe-string budget, the Czero/EECL R&D team developed our first prototype—which even made a public debut in a local parade. This pragmatic, robust solution fulfilled the project criteria for simplicity, ease of installation, price point, and supply and distribution chains.

Ultimately, though, we concluded that because of the extreme diversity of vehicles in India and many developing nations a retrofit solution wouldn’t be commercially viable. The feasibility study results and other research did, however, support commercial viability of the retrofit kit for markets with a higher number of modern vehicles. We estimated that upgrading existing vehicles in the U.S. to hydraulic hybrid systems using this retrofit kit would repay investment within approximately three-years.

Project 2: Designing and developing a custom gearbox for a hydraulic hybrid vehicle

Recognizing the need, Czero and the EECL took a clean-sheet approach and designed a gearbox specifically for hydraulic hybrid vehicles. The Colorado Governor’s Energy Office partially funded the project, which included:

  • Dynamic modeling & simulation. Using MATLAB/Simulink, we modeled the entire system, including the pumps, accumulator, energy efficiency, and mass of individual gears to optimize system design, performance and controls.
  • Mechanical design. Our R&D team performed extensive analysis to arrive at the best system configuration, using finite element analysis (FEA) to anticipate hot spots and problem areas and GD&T to reduce manufacturing costs.
  • Controls. We developed all system controls utilizing MotoTron (Woodward) control and serial communication with the engine.
  • Prototyping. Czero engaged our wide network of automotive suppliers in Colorado and Detroit to have the custom gearbox parts manufactured. The team assembled the components and installed the system into an International 4700 commercial truck
  • Testing and analysis. We tested the installed prototype on a closed track and the open road, and it operated successfully, without mechanical failure of the Czero gear box, for more than two years.


Czero | Hydraulic hybrid transmission

A clean-sheet transmission design for a hydraulic hybrid vehicle; designed, built and tested by
Czero and the EECL in a 6-month timeframe


Project 3: Developing a replacement transmission that included hybrid functionality

Based on our experience and the project requirements for cost and ease of installation, we determined that the feasibility of a hydraulic hybrid retrofit solution would be greatly enhanced by developing a design that included hybrid functionality in the transmission; in this way, our hybrid product could be incorporated simply by replacing the transmission, eliminating the need to cut into the existing driveline. To address this, Czero and the EECL developed a variation of a parallel system that resulted in a smaller, lighter, and more efficient design than a conventional parallel hydraulic hybrid, leading to multiple patent applications for this concept.


Czero | Hydraulic hybrid vehicle drop-in transmission replacement 

Transmission replacement that includes hybrid functionality. This transmission replaces the existing transmission as a one-for-one replacement, reducing size, weight and retrofit costs.

Czero’s R&D services for developing hydraulic hybrids

While Czero is no longer developing hydraulic hybrid technology for commercializing our own products, we do offer consulting services for clients developing hydraulic hybrids. We also have available for licensing the intellectual property (IP) for the transmission replacement with hybrid functionality (Project 3 above).

Contact us to inquire about IP licensing or our hydraulic hybrids R&D services.