17-08-2013, 03:33 PM
The use of Micro turbine generators in hybrid electric vehicles
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Introduction
This document briefly describes some ideas for ultra-low emission power trains using a micro turbine in a
series hybrid configuration. Over the last decade or so, there has been considerable debate in the technical
press describing the relative merits and drawbacks of parallel, series and more complex drive lines.
However, most of this debate has been based on the implication of using diesel or petrol engines as the
thermal engine source.
HILTech’s activities in hybrid systems are focused on three prime sources of power for series hybrid vehicles
they are: fuel cells - rotary engine and micro turbines. The actual power train design is dependent on a wide
number of factors and can result in complex systems. For example; under certain commercial
circumstances, we can include both a small PEM fuel cell and a thermal engine system in one vehicle.
What is a Micro turbine?
Micro turbines are small combustion turbines approximately one third the size of its equivalent diesel engine
with outputs of 25 kW to 500 kW. They evolved from automotive and truck turbo-chargers, auxiliary power
units for air planes, and small jet engines and comprise a compressor, combustor, turbine, and recuperator.
Micro turbines can run on a number of fuels which include; hydrogen, CNG / LPG, alcohol, Kerosene,
recycled oil, possibly vegetable oil all which reduces dependence on diesel or petrol. Indeed using CNG /
LPG the micro turbine is a micro gas turbine. It is the replacement of a conventional thermal engine by a
micro turbine which is the significant technical innovation described in this document.
The efficiency of turbine systems
The micro turbine has a base twenty-six to twenty-eight percent (26 / 28%) thermal efficiency however, this is
compensated by weight saving and overall hybrid system efficiency. The thermal efficiency can be increased
to typically 31/32% by using waste heat absorption technology to power air conditioning or vehicle space
heating or used to power a Rankine cycle to generate more electrical energy.
The following table (overleaf) illustrates the technology comparison between a micro turbine generator
powertrain solution against the current PEM fuel cell state-of-the-art and a conventional CNG powered
thermal engine.
Emissions and pollution
Road vehicle pollution causes at least 35% of all air pollution today, resulting in an ever growing concern by
Government and the general public alike. Air pollution is now firmly on the international stage and is
responsible for enormous costs. These are not generally accounted for because of the complexity of the
issues involved from health related issues to city infrastructure damage.
Maintenance and service
HILTech’s power train design philosophy is to design in minimum maintenance requirements. It is our policy
to minimise routine maintenance This involves using low maintenance components, service diagnostic
capability and fast “black-box service replacement” of on-board vehicle components and sub-systems.
For example: a micro turbine power train will normally have a planned life of 50,000 miles between major
services and 25,000 miles between minor intermediate services. Every fifty thousand miles the gas turbine
may require new combustion chambers which would be effected by engine change and works refurbishment.
Micro turbines have a far lower maintenance profile than their conventional thermal engine counterparts (no
oil, air filters etc.).
On-board diagnostic checks will be used for: fluid levels, state of traction and auxiliary batteries, insulation
resistance, brake condition, and wherever possible as many other sub-systems it is practical or desirable to
do so. The use of wireless LAN technology for the daily automatic transfer of diagnostic information giving
the fleet operator a review of vehicle status can further reduce time and operational costs.
The traction battery may need replacing every 10 to 15,000 miles on a service exchange basis depending on
battery chemistry. Super capacitors are expected to have a circa fifteen year life, though one or two cell
failures might be expected with such large number. Protection will ensure no single cell failure causes total
system failure.