Hydrogen Bus Technology Validation Program

This page updated November 23, 2005.

UC Davis, Institute for Transportation Studies

Hydrogen Bus Technology Validation Program

CARB Grant Number ICAT 01-7

The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Executive Summary
The California Air Resources Board has approved very strict heavy-duty emissions standards for transit bus engines. The CARB 2003 heavy duty emissions standard for NOX was 4 g/bhph. The standard for 2007 will be 0.2 g/bhph – a 95 percent reduction. Both commercial transit bus engine technologies, diesel and CNG, require significant after treatment technology to have a chance of meeting the 2007 standard.
The focus of the Hydrogen Bus Technology Validation Program ICAT project was to demonstrate that hydrogen enriched natural gas (HCNG) engines could meet this strict CARB standard.
Before the ICAT program began, Collier Technologies, Inc. modified a John Deere 8.1 lliter engine to operate on HCNG fuel. This Phase I bus showed significant emissions reductions at modest powers but was unable to meet the 2007 standard. At high power, NOX emissions were well over 1.0 g/bhph. As part of the ICAT program, Collier Technologies made further modifications to the John Deere 8.1 liter engine. These Phase II modifications involved changes to the engine control program and replacing the stock turbocharger.
The Phase II engine was tested on a dynamometer to measure both emissions and fuel economy. The testing indicated significant tradeoffs between emissions and power. If the control strategy attempted to minimize emissions, the peak power of the engine was reduced. If power was not sacrificed, the emissions data at high torque exceeded the CARB 2007 standard for NOX by roughly a factor of 2. When the engine control is set to minimize emissions, the NOX emissions were below 0.2 g/bhph (the 2007 standard) for all measured torque-speed points.
Collier Technologies, Inc. used the information gained from the Phase II engine results to develop a commercial prototype HCNG engine. They choose to modify a Daewoo 11.0 liter CNG engine for HCNG operation. The larger engine allows them to de-rate the power while still having enough for transit bus purposes. Using this engine allows them to meet the 0.2 g/bhph NOX standard and still provide adequate power. Their commercialization plan now consists of marketing the modified Daewoo 11.0 liter engine to transit agencies as a replacement for the stock engine or to bus manufacturers as the stock engine.
The ICAT program involved several other tasks in addition to the design and testing HCNG engine. UC Davis researchers developed a combustion model to understand the characteristics of hydrogen enriched natural gas combustion, a dynamic vehicle model to estimate the benefits of HCNG buses, and cost models to estimate the cost to transit agencies of owning and operating HCNG buses.
An engine model with detailed chemical reactions was developed to predict the “in cylinder” production to NOX under realistic engine conditions. The model was able to predict measured NOX values over a large range of equivalence ratios. In particular, the results indicated that there is high sensitivity of NOX to equivalence ratio. For example, the variation in NOX between equivalence ratios of 0.7 and 0.6 can vary by almost an order of magnitude. Collier Technologies runs their engine at equivalence ratios below 0.6 in order to minimize NOX.
 The dynamic vehicle model was built on the Advisor vehicle platform. The bus was modeled using input files that simulated the Unitrans bus. CNG and HCNG engine maps for emissions and fuel economy were constructed from Collier technologies dynamometer data. Standard bus driving cycles, such as the Central Business District (CDB14) and the New York Bus (NYB), were used to estimate bus performance. As expected, NOX emissions were significantly less for the HCNG bus compared to CNG buses. For the CBD14 drive cycle, HCNG bus NOX emissions were 6.6 grams/mile compared to 70.1 grams/mile for CNG buses. The fuel economy was 3.2 miles/gallon of diesel equivalent (2.7 miles/gallon of gasoline equivalent) for HCNG buses compared to 2.7 miles/gallon of diesel equivalent (2.3 miles/gallon of gasoline equivalent) for CNG buses.
A hydrogen fueling station cost model was used to estimate the cost to install and operate the hydrogen portion of a HCNG fueling station. It was assumed that transit agencies that adopted HCNG technology buses would already have CNG infrastructure in place. The cost model includes capital costs for hardware, installation costs, and operating costs including fuel (for example, natural gas for stations with reformers). Costs are near to midterm costs (0-5 years roughly). The costs were broken down into hardware, installation, contingency, energy, and fixed operating costs. The overall cost was estimated both as an annual cost over the lifetime of the station and as the cost for a kilogram of hydrogen dispensed (The energy in a kilogram of hydrogen is roughly equal to the energy of 0.88 gallons of diesel or 1.04 gallons of gasoline). For relatively small stations (100 kg/day) the cost of dispensed hydrogen was estimated at $15.04/gallon of diesel equivalent ($13.30 kg hydrogen). For larger stations (1,000 kg/day) the cost for dispensed hydrogen was $7.38/gallon of diesel equivalent ($6.53 kg hydrogen).

Funding Source

Funding Amount





NRG Tech

$  26,023

Click here for the entire final report.

ICAT Funded Projects