|The Ideal Ground Vehicle
All practical ground vehicles are propelled by mechanical power at the
wheels or tracks. So, putting aside land yachts and jet propelled speed
record seekers, there needs to be some form of transmission providing
the necessary wheel torque and speed.
All powered vehicles rely on chemical energy sources, being generally a
combustion fuel (gasoline, diesel), or electrochemical (battery, fuel
cell). A converter such as an engine or electric motor outputs
mechanical power in the form of a rotating shaft. The transmission then
translates the converter output to the vehicle requirement.
In an ideal system, by my definition, the energy converter should be
designed simply to convert the chemical energy to mechanical energy as
efficiently as possible, and with a minimum of pollution where this is
It is then the task of the transmission to to take whatever speed and
torque is output from the converter and efficiently change it to the
speed and torque required at the wheels.
Auto industry speak often talks in glowing terms of a “torquey” engine,
and the provision of a good torque band is perceived as an essential
requirement. And it is always the case that the efficiency potential of
the engine is compromised to provide the torque. To my mind this is
simply a load of old codswallop. An engine only needs torque to make up
for the deficiencies of a poor transmission, otherwise it could provide
the power at whatever torque and speed provides the greatest efficiency
in energy conversion.
The Ideal Ground Vehicle Transmission
has a continuous ratio capability sufficient to match the torque and
speed of the energy converter operating at its point of maximum
efficiency for the prevailing power demand to the requirements at the
vehicle wheels, over the full range of vehicle performance.
This is not a new idea, but I still read such statements as “diesel
engines have superior torque characteristics to spark ignition
engines”. So what! Use a better transmission and it does not matter. As
we are running out of low cost diesel fuel, this is probably a very
The Drover Transmission
Drover Transmission” is a paper describing the work of Dick Ifield
his development team in producing a hydrostatic transmission that
achieved IGVT status in 1962. It consisted of a variable hydraulic pump
driving a variable hydraulic motor, replacing the standard transmission
of a Rover motor car.
The control system held the engine at the appropriate speed for the
prevailing power demand.
Engine performance for
optimum fuel efficiency
from the "Drover
|The control curve was derived from the
BMEP map of the engine to achieve the highest possible fuel efficiency
at all power levels, commensurate with engine smoothness and
There was a
demonstrable improvement in fuel economy, even as compared with the
standard manual transmission in top gear – direct drive.
economy improvement shown by Drover transmission
And this with an engine developed to
give torque, not maximum fuel efficiency.
Launch from standstill
At the moment of
launch from standstill, the vehicle is stationary so there is no power
required at the wheels, simply the launch torque. The energy converter
should only need to provide enough power to cover the efficiency losses
in the transmission and drive train.
Geared transmissions rely on a slipping clutch or torque converter so
that the engine can be run at near maximum torque, to provide the
launch torque through the gear reduction. This is both very inefficient
and one of the main reasons that engines have to be developed to have
good torque characteristics.
An IGVT, on the other hand, provides the launch torque without any need
for engine torque. A hydrostatic transmission of the Drover type can
provide full launch torque with a torque ratio of 1:20, so that the
engine can be just above idle speed.
The hydrostatic transmission is unique in that it can provide static
torque with no dynamic losses, simply a small amount of internal
leakage and friction loss. Electric motors are faced with the
fundamental physics of torque being proportional to current. Certainly
there are things that can be done with current transforming
(cryogenics?) but the HST achieves the same with simple mechanical
At the other end of transmission ratio, level cruising at normal
highway speeds only needs a fraction of the engine power of most
automobiles, so the best fuel economy is only achieved with very high
The Rover car used for the Drover transmission needed a maximum speed
ratio of 1:2.5 to optimise fuel efficiency over the range of speeds.
This was the major factor in providing improvements in fuel economy at
Transmission ratios for
optimum engine utilisation
|Modern automobiles have a greater power
to weigh ratio, so there is an advantage with overdrive ratios to 1:4.
A Drover style transmission can provide this with about 80% overall
The combination of torque ratio at launch and speed ratio at cruise
leads to an overall ratio change of 80:1. This is basic for the IGVT
concept. Transmissions with less capability simply cannot hack it. From
my knowledge this leads to two candidates – hydrostatic or electrical.
The hydrostatic excels at the torque end while the electrical has
potential advantage at high speeds (perhaps there is a slot here for a
hydrostatic/electric split drive that takes advantage of both).
Driving without fuss
The downside of
the high overdrive transmission is that the engine is always at full
throttle, this being the condition for maximum fuel efficiency. So any
acceleration requires a change in ratio, with the engine speeding up to
provide the new power demand. This infers a time lag – sluggish
response, definitely a no-no.
This is one of the drivers towards larger engines in modern vehicles as
there is a need for spare torque, even in overdrive, if drivability is
not to be adversely affected. Perhaps the majority of modern
automobiles have a theoretical top speed of more than 200 km/hr (125
MPH); partly to provide good acceleration at more modest speeds, but
mostly to provide ease of driving with minimum fuss.
The IGVT answer to this is energy storage. A small amount of storage
can provide the energy for instant acceleration and bringing the engine
up to the new speed. Even faster response than existing fuel injected
engines. Energy storage is also a requirement for many strategies in
improving fuel economy, as will be discussed later. The IGVT definition
is extended to meet this requirement:-
Further, the Ideal Ground Vehicle Transmission
has the ability to integrate high power energy storage into the drive
Only a relatively small amount of energy is required to provide
drivability with high overdrive ratios, readily provided by a small
accumulator for hydrostatic transmissions. An electric hybrid could use
Energy storage for
improving fuel economy
It is now well
established that energy storage can be used to improve fuel economy.
Electric battery storage is an obvious first choice, but unfortunately
not ideal because of their poor performance at high power levels.
Batteries have good energy density (can store lots of energy) but poor
power density (have poor efficiency at high power). Generally the
batteries in electric hybrid vehicles store much more energy than is
required to optimise fuel economy, because the are sized to provide the
necessary power without too much loss; a very difficult compromise.
Ron Kepner’s paper (SAE Paper 2002013128) on the Ford “Hydraulic Power
Assist” demonstration vehicle shows that energy storage with hydraulic
accumulator can provide useful economy improvements, even without using
an IGVT, still using a standard automatic transmission.
The use of a full IGVT would further improve fuel economy, and over a
wider range of performance.
The next major step is to cycle the engine on and off so that it is
never idling, but only used to provide power at a level where
combustion efficiency is high. This is effectively treating the engine
like a battery, switching it off when no energy is required. However,
the energy storage system has to store more energy to provide
reasonable cycling times, and also to handle that energy hungry
auxiliary called the air conditioner. With present vehicles, being
comfortable in a traffic jam on a hot day is very inefficient in
The hydraulic accumulator becomes too large and heavy. A combination of
battery and capacitance storage is one approach. A flywheel is probably
the smallest and lightest.
Incorporating a flywheel into a hydrostatic IGVT needs a third
pump/motor unit to drive it. More complication and cost. But simple
established technology with efficient high power capability.
are basically three contenders for the IGVT:-
To my mind, the
mechanical systems are simply not up to the performance requirements of
the IGVT specification. Both electrical and hydrostatic have potential,
and it is likely that both will share some of the future. Possibly
electrical for smaller passenger vehicles and hydrostatic for
transport. Time will show.
There are many mechanical variable speed drives, such as the Van Doorn
and Torotrak. They are all limited in ratio capability.
Much research and development investment has been put the way of
electric drives on the basis that batteries or fuel cells would meet
all future requirements.
A possibility that has not been realised, nor received much in the way
of investment or grant funding, but still the simplest short term
answer, with no requirement for the development of new technologies.
Obviously, I am an advocate of the hydrostatic approach, but this will
not be achieved without serious commitment. It will not simply be a
matter of adapting existing mobile hydraulic equipment to automotive
use, as the background of development is simply to different.
As far as I know, only the Ifield design was specifically developed for
automotive applications, and it shows. The basic philosophy is
different. The concentration on part power efficiency for example, as
this is where vehicles spend most of their lives. Taking the Ifield
concept as a starting point, the challenge is to complete the journey