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The World’s Most Highly
Hybrid Petrol/Elec
Only a complete fruit-loop would modify a car with no less than six
Electronic Control Units, a 288V battery, two electric motor/generators
and a control system that frequently switches off the engine, incorporates
regenerative braking and uses electronic throttle control.
Well, welcome to that car – and the crazy modifier, regular
SILICON CHIP contributor Julian Edgar.
F
or fifteen years, modifying cars has been my passion. I’ve run 21 psi boost on a 3-cylinder Daihatsu
Mira, rebuilt a BMW 3.0si engine and played with
cars as diverse as a V8 Lexus LS400 and a turbo Nissan
Maxima V6.
For the last decade or so it’s been not only a hobby but also
a livelihood – over that period I’ve worked full-time for both
automotive print and performance on-line magazines.
But a few years ago I started getting bored. After all,
there’s only so many cars fitted with huge turbos or even
bigger exhausts you can feel excited about. I’d seen it all
before – and it no longer thrilled. Then I came across a
16 Silicon Chip
hybrid Toyota Prius at a price I couldn’t go past. A grey
market Japanese import, it was the very first domestic
Japanese model. Compared with the two Prius models
sold new in Australia (see the Prius Models panel), it had
less engine and electric power but it was still a full hybrid
petrol/electric car.
Here was a car I could really get my modification teeth
into – literally anything I did would be cutting edge, never
done by anyone else in the world.
Forget ringing up the local workshop to ask what turbo
size would be suitable; don’t bother joining an on-line
discussion group to find out what flow injectors are fitted;
siliconchip.com.au
y Modified
ctric Car
Prius Models
So what are the di
fferent Prius mod
els? Julian Edgar’s
model, the NHW
10 (pictured at le
ft), was released
Japan in late 1997
in
. It uses a 43kW pe
trol engine and ha
30kW of electric
s
power available.
The first model Pr
ius sold outside
NHW11 model wh
of Japan was th
e
ich looks the sam
e as the NHW10
has some signific
but
ant changes, inclu
ding engine powe
increased to 53kW
r
and electric powe
r increased to 33kW
The current mod
el, the NHW20,
.
has both a new
and although the
body
fundamentals re
main the same,
more significant
even
driveline change
s.
Engine power ha
s increased to 57
kW and electric po
er is up to 50kW.
wThe NHW20 has
been selling well
in Australia and ar
both
ound the world, es
pecially in the US
.
in a reduced intake charge. So despite the
geometric compression ratio of the Prius being a sky-high
13.5:1, the cylinder pressures on the compression stroke
don’t really reflect this.
This approach benefits efficiency because at lower loads,
the throttle is open wider for a given power output, so
reducing pumping losses.
To allow the degree of ‘Atkinsoning’ to be altered on the fly,
the engine Electronic Control Unit (ECU) alters intake valve
cam timing. The other vital ingredient in making this process
work is the use of an ECU-controlled electronic throttle. In
this car, the driver’s torque request often has little to do with
the actual throttle angle selected by the ECU!
Atkinson Cycle (sometimes called Miller Cycle) engines
have low-RPM torque. But in the Prius, there’s a dirty great
big 30kW AC electric motor ready to provide maximum
torque at zero road speed.
Two motor generators
don’t worry about looking around for off-the-shelf electronic
aftermarket parts.
I wanted a challenge – and this was certainly it.
Prius driveline
The Prius uses a 1.5-litre 4-cylinder engine closely related
to the engine used in the Toyota Echo/Yaris. However, it
has much less power than the Echo – just 43kW. The low
power output is because the engine revs to only 4000 RPM
and uses what is called an Atkinson Cycle.
Compared with the conventional Otto Cycle, an Atkinson
Cycle engine delays the intake valve opening time, resulting
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In fact, the gearbox (called the Power Split Device or PSD)
actually contains two electric motor/generators. Along with
the engine, these are connected to an epicyclic gear train.
The engine’s output is split between the wheels and one
of the generators. The generator charges the high voltage
battery or alternatively, feeds the other electric motor that
in turn helps drive the wheels. This electric motor can also
receive power from the high-voltage battery to either assist
the petrol engine or propel the car on its own.
The PSD’s gear ratio is a result of the balance between
the speeds of the engine, the electric motor/generators and
the wheels and that depends on how much force is applied
by each. This gives the effect of a continuously variable
transmission.
One of the electric motors also acts as a quiet and powerful starter for the engine, allowing it to be stopped and
started smoothly as needed. The other generator is used to
regeneratively recover energy from the car during braking
and store it in the battery for later use. When the driver lifts
the accelerator pedal, the engine’s fuel supply is cut off.
For more on Prius technology, see the December 2001
issue of SILICON CHIP.
Modifying the regenerative braking
One of the first electronic modifications I performed was
to increase the amount of braking regeneration.
March 2006 17
To keep the Prius feeling as conventional as possible,
both the regen and conventional brakes are controlled by
the one brake pedal. In the first part of its travel, the brake
pedal operates the regen brakes alone and as further pressure is placed on the pedal, the friction brakes also come
into play.
However, the regen braking in the Prius didn’t seem
particularly strong. That may be because I had installed
rear disc brakes and was running high-performance Kevlar
brake pads with slotted discs front and back – perhaps the
hydraulic brakes were doing more work than had been
intended by the designers.
The central colour LCD in the dash shows by means of
small ‘sun’ symbols how much regen has occurred each
five minutes, with each sun indicating a regen of 50 watthours. But in my driving, seeing a lot of suns was rare – so
it seemed to me that if the regen could be tweaked to do a
greater proportion of the braking work (especially in light
braking), fuel economy would benefit and the braking would
be smoother.
The ABS ECU handles regen braking, sending a signal to
the hybrid ECU to tell it how much regen to impose. But
how does the ABS ECU know what to do?
Rather than measuring brake pedal travel (which could
vary with pad wear, etc), the system uses sensors to detect
master cylinder pressure – the higher the master cylinder
pressure, the harder the driver is pushing on the brake pedal.
If the pedal pressure is low, the ECU knows that the driver
wants only gentle deceleration and so instructs the hybrid
ECU to apply only a small amount of regen. However, as
master cylinder pressure increases, so does the amount of
regen that can be automatically applied.
If the amount of regen that occurs is largely dictated by
the output of the master cylinder pressure sensor, why not
intercept and alter this signal? That way, the ABS ECU will
think that there is more master cylinder pressure than is
actually occurring, so resulting in more regen being applied.
Since the actual hydraulic pressure going to the brakes is
unchanged, there’ll be a greater proportion of regen braking
in the mix.
The voltage output of the pressure sensor ranges from
about 0.4-3V, rising with increasing pressure. So if a small
voltage was added to this signal, the ECU should respond
with more regen braking.
But would the ECU detect a fault condition? The workshop
Fig.1: this circuit that initially used to alter the amount
of regenerative braking. A 100kW pot was wired
between the output of the sensor and the regulated 5V
supply, allowing the signal to be increased in voltage.
To give the required fine adjustment, a multi-turn pot
was used. Note: the ABS ECU has many more wiring
connections than are shown in this diagram!
18 Silicon Chip
A colour LCD in the middle of the dash can be configured
to show economy (measured in kilometres per litre
in five minute increments) and the amount of braking
regeneration that has occurred. Here it can be seen that
over the last 30 minutes economy has never dropped
below 20 kilometres/litre (that’s 5 litres/100 km) and that
150 watt/hours of regeneration has occurred. Being driven
enthusiastically and with some hard country road hillclimbing thrown in, economy of the modified car averages
in the high fives in litres/100 km.
manual states that a fault will be detected if the voltage from
the sensor is outside the range of 0.14-4.4V, or if it is outside
a certain ratio to its nominally 5V supply voltage. Further,
the latter is checked when the brake switch is off (ie, brake
pedal is lifted). In other words, the voltage needs to be within
a certain range and in some cases this is checked with the
brake pedal not activated.
Fig.1 shows the circuit that was initially used. A 100kW
multi-turn pot was wired between the output of the sensor
and the regulated 5V supply rail, with the wiper of the pot
connected to the original ECU input.
However, this circuit did not take into account the selfchecking by the ECU of the input signal with the brakes not
applied. Fig.2 shows how a relay was wired into the circuit
so that the pot was bypassed whenever the brake pedal was
Fig.2: the circuit shown in Fig.1 did not take into
account the self-checking by the ECU of the input
signal without the brakes applied. This diagram shows
how a relay was wired into the circuit so that the pot
was bypassed whenever the brake pedal was released.
siliconchip.com.au
released. At very light pedal brake pressure, this modification
has caused the voltage at the ECU input to be lifted from 1V
to about 1.15V. This results in clearly stronger deceleration
when the pedal is lightly pressed and much stronger regen
than normal as the pedal is pressed harder. At high braking
efforts, the behaviour of the car is near standard – it’s in very
light braking where there’s a clear difference.
And the amount of regen now occurring? The regen braking is clearly doing much more of the braking work. This
can be both seen in the display of watt-hours regenerated
(there are more suns appearing) and also in the feel of the car.
The regen braking is smooth and effortless, slowing the car
substantially before the brake pedal is moved a little further
to activate the hydraulic brakes and bring the car to a halt.
It’s hard to assess overall urban fuel consumption, but in
some 5-minute increments, it has improved by 30%.
Intercooled turbo
On the road, the Prius doesn’t feel particularly slow, despite its low power and 1240kg mass. The electric motor’s
low-speed torque and petrol engine integrate seamlessly,
giving punchy performance in all but one driving situation.
That situation is climbing a long, steep country road hill.
Initial performance is fine but after a while, the high voltage
battery becomes drained, decreasing the amount of electric
power available. The car slows, the engine revs automatically increase, and you can find yourself with the accelerator
pedal flat to the floor, just crawling along.
At this point a rather cute tortoise symbol lights up on
the dash – we christened her ‘Myrtle the Turtle’.
And it just so happens that I live at the top of a long,
country road hill that has gradients as steep as 16% – and
Myrtle is a real pest.
By feathering the throttle and watching the movement of
current in and out of the HV battery (the colour LCD shows
this), it was possible to negotiate this hill with a speed
over the crest of 55km/h or so. But lose concentration and
that dropped to 47km/h. That’s awfully slow. (It should be
stressed that the later model Prius cars don’t suffer from
this hill-climbing problem.)
I considered adding another HV battery pack but because
of their control systems, this is an extremely complex move
– nothing like as simple as wiring the two batteries in parallel. That left increasing engine power as the best option and
so I turbocharged and intercooled the engine.
And then the fun started….
Fascinatingly, the hybrid control system coped with the
increased power remarkably well.
Presumably because Toyota’s engineers could never
predict exactly how much power the petrol engine would
generate (as with all engines, this varies with atmospheric
conditions, individual engine build quality and so on), the
hybrid system had enough flexibility in its control system
to direct any excess power being developed by the engine
into charging the HV battery. Note: into the HV battery, not
to the wheels…
So the odd situation developed where, with the HV battery
level at (say) half, the performance of the car was standard
– despite the turbo! But what about up the big hill?
Ah, well there the car was transformed. Rather than the
HV battery dropping in charge, going up the hill it actually
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siliconchip.com.au
March 2006 19
A thick-walled exhaust manifold was fabricated to locate
and direct exhaust flow to the added turbocharger. The
mechanical addition of the turbocharger was relatively
straightforward – it was getting the air/fuel ratio suitable
for the forced aspiration that caused Julian Edgar to pull
out his hair….
increased in charge! And always having lots of battery voltage – and so electric motor power – resulted in the speed
over the crest of the hill increasing from a low of 47km/h
to a stunning 86km/h.
But let’s go back a little. When the turbo was fitted,
the air/fuel ratios needed to be changed – a turbocharged
engine (especially one with a 13.5: 1 static compression
ratio, Atkinson or no Atkinson) needs richer than standard
mixtures. The Prius uses an air/fuel ratio of 14.7:1 all the
time – the engine ECU monitors the output of the two
oxygen sensors to constantly hold this air/fuel ratio. So
how to change this?
The first step was to fit a Digital Fuel Adjuster (from
“SILICON CHIP’s Performance Electronics for Cars”) to allow
alteration of the airflow meter output.
However, monitoring the mixtures with a MoTeC air/
fuel ratio meter showed that the Prius ECU is extraordi-
The Prius airflow meter sensing element is normally built
into the airbox. With the addition of a turbo, the standard
airbox could no longer be fitted and so the airflow meter
sensing assembly was removed and built into this new,
larger airflow meter. The SILICON CHIP Digital Fuel Adjuster
(DFA) was then used to correct the output signal.
20 Silicon Chip
To provide the additional fuel required for the turbo, a
dual-pressure fuel system was installed. This uses two
pumps, two pressure regulators and a solenoid to switch
from low to high fuel pressure. The switching is done by a
SILICON CHIP Simple Voltage Switch that monitors airflow
meter output voltage. When load rises sufficiently, the fuel
pressure is switched high and at the same time, the twin
oxygen sensors are disconnected. This approach prevents
the engine Electronic Control Unit ‘learning around’ the
changed mixtures.
narily quick at learning around any changes made in this
way. Alter the mixtures to 12.5:1 and within five or so
seconds, the mixtures are back at 14.7:1! The same thing
occurs if fuel pressure is increased – back go the mixtures
to stoichiometric.
Hmmm, so what about disconnecting the oxy sensors,
substituting an appropriate looking 0-1V square wave signal
on the ECU oxy sensor inputs and then altering the airflow
meter output voltage?
A pair of 555-based circuits was constructed and the
system wired-up. But the ECU immediately picked that
something was wrong with the oxy sensors and went into a
default mode – which with the added airflow of the turbo,
resulted in mixtures even leaner than 14.7:1! Aaaaagh.
Well then, what about disconnecting the oxy sensors
(easily achieved just on high load with the “Simple Voltage
Switch” [also from SILICON CHIP Performance Electronics
This 555-based circuit was built in an attempt to simulate
the fluctuations in oxygen sensor voltage normally seen by
the Electronic Control Unit (ECU). If successful, this would
allow the oxy sensors to be disconnected, stopping the
ECU learning around mixture changes. However, the ECU
immediately saw through the pretence, outputting the same
mixtures as when there is no oxy sensor input at all.
siliconchip.com.au
These logged traces compare the input and output of the
brake pressure modification circuit. As can be seen, the
output trace shows a lot more ‘area under the curve’,
indicative of a higher brake pressure being monitored
by the Electronic Control Unit. The result is increased
regenerative braking which improves economy and
reduces friction brake wear.
for Cars] working off the airflow meter output voltage)
and then using the Digital Fuel Adjuster (DFA) to alter the
mixtures? Again, no success – and this time, the mixtures
appeared to vary randomly.
About this time I upgraded the fuel system with a new
in-tank pump, external adjustable pressure regulator and a
return-line to the tank. That allowed me to run higher fuel
pressure (which initially gave correct mixtures with the
oxy sensors disconnected) with the DFA used to tweak the
resulting mixtures. But, yet again, the mixtures were not
consistent.
After many weeks of work, I finally devised an effective
system. Two fuel pumps and two fuel pressure regulators
are used to allow the running of two different fuel pressures. A solenoid allows electronic switching between the
two different pressures. The lower of the two pressures is
set so that, even when running a little turbo boost, the ECU
can keep the mixtures at 14.7:1 and the oxy sensor feedback
loop operates as normal.
Then, when a preset load is reached, a Simple Voltage
Switch monitoring the airflow meter voltage switches out
the oxy sensors and activates the solenoid increase in fuel
pressure. The resulting mixtures are then fine-tuned by a
DFA working on the airflow meter output.
With this approach, the mixtures are consistent and economical (the car is still in closed loop with 14.7:1 mixtures
for the vast majority of the time) but with appropriately rich
mixtures used at full turbo boost.
(Incidentally, ignition timing has never been a problem. I
have never heard the engine detonate, even when [briefly!]
running 15 psi boost. 95 octane fuel is used – the car is
designed to run on 91.)
Auto throttle shutdown
About this stage I started to relax. Ahhh, this is nice...the
world’s only turbocharged, intercooled Prius with modified
regen braking. Those many hours of work were well worth
it. And that was the case until I discovered that the hybrid
control system’s adoption of turbo power wasn’t as seamless
as I’d first thought.
Initially, I’d decided the abrupt engine shut-down that
occasionally occurred at full power was an ignition problem
– and had replaced the spark plugs with a colder heat-range
Iridium design.
siliconchip.com.au
March 2006 21
But then, while watching the boost gauge, I saw what
was happening. At full power, the hybrid system would
momentarily close the electronic throttle. Whether that’s to
protect one or both of the electric motor/generators – or for
some other reason – I still don’t know. But the result was a
huge power loss perfectly timed to occur when overtaking
a semi-trailer….
If the shutdown was a result of excessive engine power, I
could just drop turbo boost at higher revs. But the problem
was that I couldn’t. The waste gate spring pressure on the
turbo meant that 7 psi was as low as I could go.
But there had to be another way of dropping boost – and
there was. By using a solenoid to control the boost pressure
feed to a recirculating blow-off valve, the valve could be
made to leak, bleeding boost from the compressor outlet
back to the inlet. The result is decreased boost.
I initially used the Performance Electronics for Cars Independent Electronic Boost Control (IEBC) to achieve this
function but ran into a snag. The IEBC allows the mapping
of a pulse-width controlled solenoid on the basis of engine
load, as measured by injector duty cycle.
However, it doesn’t have an adjustable hysteresis function
and so the solenoid would close (causing the blow-off valve
to open) and then as a result of the lower boost, engine load
would drop, thus switching the solenoid back on! Boost
would then surge up and down.
(This isn’t normally a problem with the IEBC because
the slope of the adjustment curve can be made gentle. But
in the case of the Prius, it had to be a much more sudden
change.) The answer was to (again!) use that most ubiquitous of building blocks, the Simple Voltage Switch. With
its adjustable hysteresis, it was ideal in this application,
wired-in to monitor airflow meter output signal.
As finally configured, boost rises to 7 psi and then above
the preset load point, smoothly drops to 4 psi. This makes
no difference to on-road performance – and there are no
throttle shut-downs!
Auto engine off
While compared with many turbo applications the Prius
turbo is not working particularly hard, there is one characteristic of its hybrid control that if left unaddressed, could
quickly kill the turbo bearing.
As described earlier, when the throttle is lifted, fuel flow
to the engine is stopped. So, approaching a red traffic light,
the engine stops running as soon as you back off – and stays
off until the lights go green and you apply the accelerator.
A turbocharger relies on engine oil flow for lubrication and
partly for cooling, and so the engine should not be turned
off until the turbo has had time to cool. If the turbo has been
spinning hard, an early engine switch-off can cause oil in
the turbo bearing to coke.
So how could the Prius petrol engine be kept running
after a boost event?
This model Prius has two air-conditioning modes. In
High mode, the engine is forced to run continuously. In
Normal mode, the engine is allowed to switch off whenever
the hybrid ECU decides it should be off. When High mode
is selected, the air-con system tells the hybrid ECU that it
should not switch off the engine by means of an ‘engine
on’ request signal. This signal is very simple – above 4V
means keep the engine running, below 1V means it’s OK
to switch it off.
22 Silicon Chip
To prevent the hybrid control system automatically
switching off the petrol engine shortly after the turbo has
been on boost – and so starving it of oil – a Simple Voltage
Switch was modified to provide the ‘engine on’ request
signal than normally occurs when the air-conditioner is
switched to high. This photograph shows the new MOSFET,
capacitor, diode and resistor that allows the Simple
Voltage Switch to feed the correct signal to the hybrid ECU
for 30 seconds after the turbo has been boosting.
So by feeding 5V to the ‘engine on’ input of the hybrid
ECU, the engine can be kept running.
Yet another Simple Voltage Switch was used to monitor
the airflow meter signal, sensing when engine load was
above a certain threshold that corresponded to a few psi of
boost. To derive the required 5V signal to feed to the hybrid
ECU, a pot was placed across the regulated 8V supply used
in the Simple Voltage Switch. The Simple Voltage Switch
was then modified so that once it was triggered, it stayed
on for about 30 seconds.
So how well does the system work? In most cases the
delayed ‘on’ time for engine running isn’t noticeable – when
the car is moving, it’s hard to tell whether the engine is firing
or not. But if the car is being driven hard around the city,
pulling-up at a red traffic light causes the engine to keep
running for a short time, when previously it would have
switched off as soon as the car was slowing for the stop.
Conclusion
We’ve run out of space to cover all the mods made to the
car – they include a rear sway bar, electronically modified
electric power steering, on-dash mixture indication, electronically controlled high pressure intercooler water spray,
under-floor aerodynamic changes and plenty of other bits
and pieces.
And the results? Well, the worst aspect of the car was
previously its country road hill-climbing ability – and that’s
been improved by over 80%!
But what about the raison d’etre of the Prius: fuel economy? The modified Prius now has better than standard fuel
economy.
On an open road cruise at 100km/h, the turbo Prius will
turn in a best economy of about 5.5 litres/100km, whereas
in the same conditions, the dead standard car used to get
about 6.3 litres/100 km. And even when being driven hard,
the economy now averages about 5.8 litres/100km.
SC
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