1. Split Hybrid
A New Development in Hybrid Engines
© 2001 DEMACO Engineering Services.
The information contained in this document is confidential and the property of DEMACO Engineering Services. No part of this
document may be copied or otherwise used without the express permission of DEMACO Engineering Services. The copyright and
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DEMACO Engineering Services
United Kingdom

2. DEMACO Split Hybrid
Contents
Contents..................................................................................................................................2
1 Introduction............................................................................................................................3
1.1 The Hybrid concept.........................................................................................................3
1.2 Main benefits of a Hybrid system....................................................................................3
2 The Split Hybrid.....................................................................................................................4
2.1 System Components.......................................................................................................4
2.2 Hybrid concepts...............................................................................................................5
2.2.1 Series Hybrid............................................................................................................5
2.2.2 Parallel Hybrid...........................................................................................................6
2.2.3 Split Hybrid................................................................................................................7
3 Split Hybrid - Modes of Operation..........................................................................................8
3.1 Modes of operation overview...........................................................................................8
3.2 Urban Mode.....................................................................................................................9
3.3 Steady State Cruise Mode.............................................................................................10
3.4 Full Power Mode – Constant.........................................................................................11
3.5 Full Power Mode- Burst.................................................................................................12
3.6 Deceleration Mode........................................................................................................13
4 Physical realisation..............................................................................................................14
4.1 System Component Specification..................................................................................14
4.2 Heavy Goods Vehicle....................................................................................................14
4.3 Consumer vehicle..........................................................................................................14
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3. DEMACO Split Hybrid
1 Introduction
1.1 The Hybrid concept
The concept of a Hybrid Powered vehicle is mature to the extent that production models exist.
Hybrid Power, whilst addressing the need for enhanced fuel economy, does have some
limitations. DEMACO Engineering Services have developed the Hybrid Concept to reduce or
eliminate these negative effects and therefore further increase efficiency.
In order to offer maximum efficiency across the range of power demanded of an automotive
engine, DEMACO have developed the concept of the Split Hybrid. In this method the benefits
of conventional hybrid designs have been combined, whilst the negative effects have been
reduced.
The key to these enhancements is through engineering a system that reacts to several
variables and operates in the appropriate mode for maximum efficiency. The Split hybrid
operates in Five distinct modes, offering optimum fuel use for each set of operating
conditions.
The key feature of the Split Hybrid is an electronically controlled intelligent clutch system. The
clutch enables the physical change between Series and Parallel modes to ensure that the
energy within the system is being delivered at optimum efficiency to the road or between
subsystems.
1.2 Main benefits of a Hybrid system
The most important factor in producing a hybrid power source is efficiency. In addition to this
the Split Hybrid offers other advantages;
• Reduction of emissions through less fuel usage and more efficient burning of fuel.
• Ability to operate in Zero Emission areas by running on battery power only.
• Designed to be manufactured using readily available subsystems – no “exotic”
technology.
• Matched system components suffer less fatigue therefore reducing service costs.
• End unit prices comparable or less than current internal combustion engine
systems.
• Future proof design. The small internal combustion engine can be easily removed
and replaced with another power source, for example a Fuel Cell. This
overcomes the legacy problem that might be encountered as emission targets are
further reduced. The end users investment is protected.
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4. DEMACO Split Hybrid
2 The Split Hybrid
In order to understand the Split Hybrid concept, it is first necessary to consider the various
components of a system and the conventional Hybrid designs; Series and Parallel.
2.1 System Components
A hybrid vehicle is one that has two power supplies; the most common combinations include
petrol/electric and diesel/electric. The two power sources can operate independently or
together depending on the circumstances. The following components are common to all
hybrid systems.
• Internal Combustion Engine (IC engine)
Burns vaporised liquid fossil fuel, under pressure, to create rotational energy. In
Hybrid Vehicles the IC engine can be smaller than required in conventional vehicle
because it is only required to operate in a narrow power band at a constant speed.
The IC engine runs at its highest level of efficiency resulting in low running cost and
low emissions
• DC Generator
Converts rotational energy into DC electrical energy.
• Battery
Stores energy by converting DC electrical energy into chemical energy. Produces DC
electrical energy from Chemical energy. Inverter
Electronically converts battery power into high energy electric power in order to drive
the Electric Motor
• Electric Motor
Converts electrical energy into rotational energy.
• Transmission
Transmits rotational energy through a series of gears to the drive wheels
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5. DEMACO Split Hybrid
2.2 Hybrid concepts
2.2.1 Series Hybrid
The Series Hybrid is, as it’s name suggests, a line of components. The Internal Combustion
engine drives the generator which charges the batteries. The output of the batteries is used to
power the electric motor.
In this arrangement the battery is always powering the vehicle. Variations in driving load are
accommodated by the battery/Inverter/motor combination, consequently, the engine is only
required to run at a constant speed, so it can be made to operate at its highest efficiency level
with minimum emissions that fit comfortably inside the limits for an Ultra Low Emissions
Vehicle (ULEV).
The major limitation of the Series Hybrid is the lack of sustained peak power. This
configuration is more suitable for daily commuting trips of less than 25 miles with an
occasional longer journey.
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6. DEMACO Split Hybrid
2.2.2 Parallel Hybrid
The Parallel Hybrid allows the IC engine and the electrical motor to work simultaneously (in
parallel). A small IC engine can be used because additional torque required for acceleration is
provided by an electric motor. When the engine turns the drive-shaft, it also spins the motor's
rotor. The motor becomes a generator, which can charge the batteries.
As for the Series Hybrid, the parallel system is efficient because the IC engine can operate in
an efficient power band, with additional energy being supplied on demand by the electric
motor. But the parallel hybrid suffers from not being able to run the IC engine at a constant
speed since it its connected directly to the transmission, therefore efficiency is not optimum.
A parallel hybrid is preferable where bursts of speed are necessary, for instance motorway
driving and country road use where steep gradients are encountered.
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7. DEMACO Split Hybrid
2.2.3 Split Hybrid
Outline
This arrangement combines technology from both series and parallel systems and arranges
them to gain the maximum benefits from each hybrid configuration i.e. the advantages of a
series system on an urban cycle and the advantages of a parallel system for highway use.
When this system is operating in a parallel configuration it has an additional advantage over
the standard parallel. The IC engine of the standard parallel is required to operate over a
broad speed range with the electric motor providing additional torque when necessary,
therefore some efficiency is sacrificed when compared to an IC engine which is only designed
to operate within a narrow band. The IC engine in the Split Hybrid is only intended to provide
power to sustain a given speed, so can be approximately 30% smaller than standard parallel
IC engine.
The Split Hybrid configuration consists of a turbo charged diesel IC engine, whose output
shaft is clutched to the through-shaft of an electric motor but directly linked to a three phase
generator. The generator continuously charges batteries and, via an intelligent control unit
and inverter, drives the electric motor, which in turn is connected to the wheel axle.
Operation
When speed requirement is low and/or power demand is high, the IC engine clutch is
disengaged, leaving the vehicle driven solely by electric power. It is important to note that the
batteries are still being charged by the generator via the IC engine, even though the engine
itself is not being used to drive the vehicle.
As demand for power reduces, i.e. when cruising speed is attained, the control unit will
accurately match the IC engine speed to the motor shaft and engage the clutch. At this point
the electric motor shaft is acting purely as a lay shaft between the IC engine and the wheel
axle. Power to charge the batteries is still supplied from the generator at this time to ensure
full battery power is available when required, the generator load is regulated by a burst fire
power controller which enables the control system to effectively split the IC engine power
output. If at any time during the steady cruise state power for accelerating or climbing is
required, this is provided by introduction of the electric motor when the control unit detects
demand.
The control unit continually monitors all parameters associated with the system and
depending on demand, will select or combine the various operating modes to ensure optimal
efficiency is achieved at all times.
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8. DEMACO Split Hybrid
3 Split Hybrid - Modes of Operation
3.1 Modes of operation overview
The Split hybrid has five distinct modes; Urban, Steady State Cruise, Constant Full Power,
Burst Full Power and Deceleration. The control systems ensure that the appropriate mode is
always selected for maximum efficiency for the particular driving conditions. The switch
between modes is made rapidly and smoothly due to intelligent use of control and feedback
throughout the vehicle.
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9. DEMACO Split Hybrid
3.2 Urban Mode
The IC engine is mechanically de-clutched from the transmission. The electric motor is
providing all motive force necessary to propel the vehicle. All the energy required for the
electric motor is supplied by the batteries. This mode of operation has some similarity to the
Standard Series Hybrid.
In this mode the IC engine has two possible states; Standby and Operating speed.
Operating speed is set by the control system if the batteries become discharged to less than
40% capacity. The burst Fire Power controller loads the Generator to provide electrical power
to charge the batteries. When the Batteries have reached 95% charge, the IC engine is set to
standby speed. (Charge level is not set to 100% because some reserve charge capacity
would be required if the system went into Deceleration Mode.).
Standby speed will be approximately 33% of operating speed, sufficient to power ancillary
engine and vehicle systems, such as; air conditioning, power steering, lighting, etc.
While this charge/discharge process (cycling) continues the variable power levels for
propulsion of the vehicle are being met by the electric motor. The output power of the motor is
being directly controlled by the Variable Frequency Drive (VFD) which is drawing directly from
the batteries.
Note; Lead acid battery life decreases exponentially with depth of discharge. For this reason
the values of 40% and 95% for charge thresholds are chosen to reduc the number of cycles in
the lifetime of the batteries.
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10. DEMACO Split Hybrid
3.3 Steady State Cruise Mode
The IC engine is operating at approximately 80% of rated output with power being split 70%
to the transmission and 10% to battery charging.
The control system will select this mode if the transmission speed (ie road speed) ideally
matches the optimum output speed of the IC engine, typically 3000rpm, or about 60mph in
fourth gear. At this point the vehicle will be cruising, with zero acceleration, in its most efficient
mode.
In this mode the IC engine is providing all of the motive force to propel the vehicle, with any
spare capacity being used to charge the batteries. When the batteries reach a fully charged
condition the proportion of IC engine power which was being used to charge them will be
removed (accomplished by the burst fire power controller going to zero output).
During the steady state cruise mode the electric motor is “freewheeling”. There is no power
being supplied to it from the VFD. While the electric motor is freewheeling its sole task is to be
a mechanical connection between the clutch and the transmission.
If additional power is required to maintain vehicle speed the system will switch to Full Power
Mode for as long as the additional power is required or the battery charge level falls below a
predetermined level.
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11. DEMACO Split Hybrid
3.4 Full Power Mode – Constant
The IC engine is mechanically de-clutched. The electric is motor producing up to its maximum
rated power and providing the entire motive force necessary to propel the vehicle. This mode
of operation is very similar to the urban cycle but the control system will monitor the rate at
which power is being used from the batteries and will put the IC engine and generator into
maximum charge condition before the batteries reach their minimum charge threshold.
The duration of Full Power Mode is a function of battery capacity plus percentage of full load
that the generator is capable of supplying.
(For approximation, the higher the duty cycle the larger the IC-generator combination. A 410-
hp tractor unit would require an IC engine of approximately 180hp.)
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12. DEMACO Split Hybrid
3.5 Full Power Mode- Burst
The IC engine is operating at its most efficient speed, as in Steady State Cruise Mode.
If additional power is required, for example during overtaking or on encountering a steep
gradient, the electric motor (which was operating as a lay shaft) will take power directly from
the batteries (via the VFD) to provide the additional power required. The IC engine and the
electric motor are working in parallel.
This mode will be maintained as long as the extra power is required AND the IC engine can
operate at its most efficient speed. If the speed drops outside the efficient operating range of
the IC engine it is de-clutched and the system transfers to Constant Full Power mode, ie
electric motor only.
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13. DEMACO Split Hybrid
3.6 Deceleration Mode
The IC engine is mechanically de-clutched. The generator load is governed to 0% and the IC
speed set to standby.
The control system will respond to the request for breaking torque by putting the VFD into
regenerative mode, this causes the Electric motor to act as a generator. The resulting power
is sent to the VFD which in turn directs it into the batteries.
In addition, the load on the electric motor in its generator mode will assist in slowing the
vehicle.
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14. DEMACO Split Hybrid
4 Physical realisation
4.1 System Component Specification
4.2 Heavy Goods Vehicle
4.3 Consumer vehicle
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