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This article was first published in 2005.
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Over the last 100 or so years, a number of alternatives to the reciprocating
internal combustion engine have come and gone.
Electric cars were once more popular than petrol engine cars (and electric
cars held the first speed records!); the rotary engine is now championed only by
Mazda but once nearly every major car manufacturer in the world held licenses
for its production (remember the rotary Corvette and Mercedes C111 prototypes?);
and turbine cars were developed by both Rover in the UK and Chrysler in the US.
But compared with the incredible number of piston engine cars that have been
produced, none of these alternative powerplants can be viewed as having been
successful.
The same may happen with hybrid cars like the Toyota Prius – those that
combine a petrol engine with a battery electric system. Long-term, they may go
the way of the turbine car. Or perhaps they’ll be more like the rotary – still
being produced, but for only a niche market.
However, our feeling is that hybrids may well be more than that. Either as a
stepping stone to fuel-cell vehicles or as a long-term sea-change in their own
right, we think hybrid cars are here to stay.
And modifying hybrids? Well, that’s on the very leading edge of car
tweaking.
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The thing about doing what no-one else has done is that there’s no obvious
source of advice.
When I bought my Japanese-import ’99 Toyota Prius, it was with the intention
of making some fairly major modifications. That could involve increasing the
capacity of the battery pack (perhaps the nearest analogy would be fitting a
larger nitrous tank), altering the air/fuel ratio of the engine to provide more
power-friendly mixtures, or even turbocharging or supercharging. And while
someone has turbo’d a hybrid Honda Insight, no one had performed a modification
of this complexity on a Prius.
The first engine modification step was to alter the air/fuel ratio at high
loads, a process covered at
Altering Closed Loop Mixtures. That resulted in a slight increase in power (long-term it proved less
impressive than described in that article), and also showed that the mixtures
could be changed at will once the car had been forced out of closed-loop.
About this time a half-cut Prius – complete with high voltage battery pack –
was purchased. That then gave the option of adding battery capacity, but the
battery pack is an unwieldy and complex thing, with lots of internal control
systems and circuit breakers. Certainly, adding battery capacity is nothing like
as simple as just connecting the two packs in parallel. Also, the battery pack
outputs over 300 volts DC – potentially lethal if not handled very
carefully.
Driving the Prius for thousands of kilometres also showed that the battery
pack and electric motor worked very well in providing short-term power bursts.
Especially when accelerating away from a standstill (electric motors have peak
torque at zero rpm!), the electric assist was effortless and surprisingly
strong. (Surprising, that is, for a car of this much weight – 1240kg – and this
much power - 43kW petrol engine and 30kW electric motor.) In fact the
performance downer was really only when the battery pack was short-term
exhausted – then the little petrol engine simply didn’t have enough power.
So if increasing the high voltage battery capacity was pretty difficult, what
about increasing the engine’s power output by forced aspiration?
Complexities
The Prius engine is a 1.5-litre four cylinder that’s based on the Echo’s 1NZ
block. Up top, however, there’s a different intake manifold, electronic throttle
and other detail differences.
Most important of these differences is that the Prius engine uses what is
called an Atkinson cycle. In this approach to valve timing, the inlet valve
stays open for a long time – in fact, even as the piston is well into the
compression stroke. This forces some of the intake mixture back out into the
intake manifold and so reduces the amount of charge that is trapped in the
chamber. The effective compression ratio is therefore much lower than the 13.5:1
mechanical compression ratio would indicate. However, the expansion cycle (ie when the mixture
is being burned and the piston pushed down) remains at 13.5:1, which has
benefits for efficiency.
Another odd aspect of the engine is its 4000 rpm redline, at which both peak
power and peak torque are developed.
But the most complex part of the driveline is the way in which electric and
petrol power are combined. The ‘gearbox’ (called the Power Split Device)
contains two electric motor/generators connected to an epicyclic geartrain. The
engine 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
that depends on how much force is applied by each. This gives the effect of a
continuously variable transmission (CVT).
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 recover energy from the car during braking, and store it in
the battery for later use.
When the driver lifts their right foot when travelling slowly, the engine
switches off.
Read it all quickly and the implications of forced aspiration aren’t
particularly clear. But take just these questions:
How does an Atkinson Cycle engine respond to forced aspiration? (The Eunos
800M runs a supercharger with an Atkinson cycle engine – but what is the Eunos’
effective compression ratio?)
For forced aspiration, is the compression ratio of the Prius regarded as
13.5:1 - or much lower? If lower, how much lower?
If it is regarded as much lower, is the ignition timing very well advanced to
provide as much power as possible with the low combustion pressures? With forced
aspiration, would this lead to early detonation?
If the engine power is increased, will the power split device still be able
to function?
If not, will the control system shut the (electronic) engine throttle, so negating the
effects of forced aspiration?
In that case, could just the low rpm torque be boosted (ie fatten the torque
curve while leaving peak power unchanged?)
If a turbo is used, how will its bearing be cooled if the engine
automatically switches off immediately after a full throttle event (as would
occur when driving slowly down the other side of a steep hill that’s just been
climbed)?
Boost Choices
Of the choice between a supercharger and turbocharger, a turbo gives the
highest efficiency. This is because it uses heat that’s otherwise wasted out of
the tailpipe, whereas a supercharger is drawing power directly from the
crankshaft. So in terms of fuel economy, a turbo is a better choice.
In order that the turbo could survive all the hot engine shut-downs, a
separate turbo oiling system could be used. This would use a small high pressure
pump, and dedicated oil cooler and reservoir. In addition to the convenience of
not requiring that the sump be removed to fit a turbo oil drain line, this would
also allow the turbo to be mounted much lower than normal if required.
Alternatively, a system could be configured that prevented the engine turning
off in some conditions. This could be done by accessing an ECU input from the
air con. The air-conditioning system in the Prius has two modes. When in high
power mode, the engine doesn’t switch off. This input to the ECU could be
accessed and a timer and relay used so that whenever the engine load had been
high, the timer caused this ‘engine on’ request to occur for the next minute.
That way, the engine wouldn’t ever turn off directly after a boost event.
However, the major benefit of using a turbo over a supercharger is that the
turbo boost can be easily mapped over the engine operating range. By using the
Independent Electronic Boost Control kit (see the AutoSpeed shop), the turbo
boost level could be set at each engine load point. This means that if the power
split device control system has problems with extra engine power at high rpm,
the boost curve could be tailored very accurately to take this into account.
The downsides of turbocharging? Firstly a very small turbo is needed – eg of
a Japanese Kei class car or something like the Garret GT12 ball bearing turbo.
And secondly, the exhaust manifold on the Prius faces the firewall and the top
of the engine is also tilted in the same direction – making access very
difficult, especially when working at home without a hoist.
A supercharger is much easier to fit. Because of the strange shape of the
inlet manifold, there exists space for a small blower to one side of the
throttle. In this position, the supercharger can be driven by a modified version
of the standard belt drive. A supercharger also develops more bottom-end boost
than a turbo, which may be important in the Prius if the peak power of the
petrol engine cannot be increased without upsetting the power split control
system. Compared with a turbo, a supercharger should also be able to be better
matched to the engine by way of pulley diameter changes.
It also seemed to me that the supercharger boost could be mapped by using the
Independent Electronic Boost Control to control the action of a large bypass
valve, one that would direct outlet boost pressure back to the inlet. This same
bypass valve could also act as a closed-circuit blow-off valve and to control
when the supercharger stopped bypassing and started boosting. However, I have
never heard of anyone else using this approach and it may prove very wasteful of
power.
However, the choice was finally made on practical grounds – I found a small
secondhand AMR300 supercharger available at the right price...
Next week – installing the blower and the first on-road
testing
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