Engines & Power Delivery
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’ll start this
discussion by stating that every engine, be it petrol or diesel, is a
compromise of some sorts.
Some engines
are more compromised than others, some appear so good
that a compromise cannot be easily found.
But all are a compromise.
There is
no such thing as a “perfect engine” for a few reasons, but the main one is that
one person’s idea of perfection is another person’s imperfection.
An engine needs
to be several things. It needs to be
reliable, tractable, smooth, quiet, lightweight, powerful and economical, all
at the same time. It should be rewarding
to work hard, and unobtrusive and muted when we want
to feather around. Ideally, it wants to
have a long service interval and a very long service life. It wants to be easy to service and repair,
should the worst happen.
Of course,
there are other compromises within a car that have a direct influence on the
engine. The main one is the car’s
gearing – this plays a major part and even the most flexible engine can give
disappointing performance when mated to a gearbox with too long ratios, or too
wide a gap between individual gears. By
the same token, a “peaky” engine can work very well with a close ratio
gearbox. A well set up set of ratios can
make the difference between a car that is good to drive, and a car that is
great.
Another factor
is aerodynamics, since a car with efficient aerodynamics will be easier to
accelerate at higher speeds.
The engine
design also plays an important part.
This is not the place to start discussing the valves per cylinder,
camshaft profile and valve duration, induction and
exhaust systems, fuelling, knock-sensors (and many more). However, we can make some sweeping
generalisations. In general terms, the
more valves per cylinder, the better the engine can “breathe” - that is, take
in oxygen. The more oxygen that can be
taken in by the engine, the more power it can produce (as well as other
benefits, such as a theoretical cleaner burn). Valve duration (the time that a valve is
open) is another important point - modern engines are designed with much
stricter emissions than previous units, and so have to have their valve
duration shortened to reduce emissions.
The shorter the valve duration, the more that must happen in a very
short space of time.
Until the last fifteen years or so, most
typical mass produced engines had a single overhead camshaft and two valves per
cylinder, with double overhead camshaft, four valve
per cylinder engines reserved for sportier models. These earlier “multivalve”
engines tend to be optimised to perform at higher engine speeds, and as a
result felt comparatively weak lower down the rev range. One example is the mark two Golf GTI. Under around 4,500 rpm, the 16v version feels
relatively sluggish whereas the 8v version feels peppy. Above this engine speed, the 16v version
feels quicker. Both engines perform to a
very similar level at low engine speeds, and the 16v version is only able to
utilise its greater power and torque at higher engine speeds.
However, now
that manufacturers have taken the time to develop these engines, a modern multivalve can be tuned to deliver strong power and torque
throughout the rev range. Some modern
single overhead, eight valve engines are also tweaked to produce power at the
top end of the rev range, rather than the more traditional way that these
engines work, which is providing low down heave and mid range sparkle but at
the expense of top end poke.
For on-road
use, the best engines deliver decent low down heave, but coupled with top end
power. In other words, you get good
acceleration from moderate speed in the upper gears, and sparkling acceleration
in a lower gear or two. It is getting
easier to find engines with these characteristics - drive a 2001 Focus 1·6 and compare it with a 1991
Escort 1·6 and you will see what I mean.
Despite residing in a larger, heavier car, the newer 1·6 car is quicker,
smoother, more economical and has much better low down and mid range
acceleration. Yet the newer engine is a
double overhead camshaft donk with sixteen valves,
compared to the older single overhead camshaft engine with eight valves.
Some turbodiesel owners love to talk the torque. “It’s
got loads of low down torque,” they cite, not really understanding what
that means. Power is a function of torque, torque is a function of power. If you ever look at a power and torque plot
the brake horsepower (power) figure
must cross the foot pounds (torque)
figure at 5,252 rpm. This is because:
Bhp = (Lbsft x
engine speed) / 5,252
and
Lbsft = (bhp x 5,252)
/ engine speed
Thus, if our DOHC
VTEC [link] petrol engine produces two hundred brake horsepower at 8,500 rpm, what
is the torque output?
(200 x 5,252) / 8,500
= 123∙6 lbsft
If our turbodiesel owner proudly tells us that his engine produces
two hundred foot pounds of torque at 2,000 rpm, it is producing:
(200 x 2,000) /
5,252 = 76∙2 bhp
Engine Examples
Ford’s humble 1·3 Endura-E engine, as used in
the Ka, is a good example of a traditional, “old school” engine. It is a pushrod design, driving two valves
per cylinder (for a total of eight valves). The engine uses no clever trickery to
produces a humble 59 PS and 105 Nm.
However, whilst the engine will rev to only 5,850 rpm (at the
limiter; maximum power is produced at 5,000 rpm), it delivers solid low down
acceleration – peak torque is produced at just 2,500 rpm. When extending the engine, it never feels
especially eager, and starts to wane a little at higher engine speeds. Even so, for a town-based car, the engine
works well. The primary reason why it is
universally disliked within the Ka Klub
isn’t because many members are clueless but is because it is simply not
powerful enough for the chassis. But
then Ford would point you towards the Puma...
The 1·7 litre engine in the Puma
is a variant of the Zetec-SE family, but features variable valve timing,
which enhances the output of the engine across the entire rev range. This engine delivers good acceleration from
just over 2,000 rpm, in a similar vein to the Ka, but is also keen to rev. Very keen to rev. It’ll spin into the red in at least the
bottom three gears, and I’ve not tried the top two. The Puma’s engine is so flexible that it gets
my vote for the best engine / car compromise that I have driven. Indeed, as a package, the Puma is excellent,
but complemented by an excellent engine!
Honda’s SOHC VTEC engines
are similar to the 1·7 Zetec-SE engine. Again, variable valve timing is used to
smooth the torque curve. The SOHC VTEC
has a broad power band from 2,500 rpm up to the limiter, typically between
6,500 and 7,000 rpm.
The Alfa Romeo 1·6 Twin
Spark engine is from the old school of sixteen valvers. This engine thrives on revs, so much so that
it feels limp under around 4,000 rpm, but has a wonderful, zesty nature from
4,000 rpm as it spins into the red. It
produces 120 PS, a respectable figure if not class leading, but it sounds
superb.
This compares with Ford’s
standard, belt-driven 1·6 litre Zetec-SE engine (as used in the Puma,
Fiesta and Focus),
again with double overhead camshafts and sixteen valves, and offering around
100 PS (depending on the application).
Whilst not as powerful as the Italian twin spark engine, the Zetec-SE is smoother, more economical and offers much more
low down heave. Ford introduced a chain
driven variable valve timed 1·6 litre Zetec-SE with
over 110 brake horsepower but at the time of writing I’ve yet to try it for
longer than just a few minutes.
The early Peugeot 106 Rallye used a
single overhead camshaft, eight valve 1·3 litre engine, tuned to produce 100 PS
and 108 Nm. This engine needs lots of
revs to perform (actually, it needs thrashing) despite the traditional
configuration.
The latest BMW 3-series
four cylinder “Valvetronic” engines are similar to
the 1·7 litre Zetec-SE donk,
in that they deliver good low down torque combined with good top end poke. Like the Puma, they are also clean and
efficient. Unfortunately, for BMW, the “Valvetronic” engines’ biggest weakness is no fault of their
own, it’s the larger capacity six cylinder engines’
fault!
Ka Hill Climbing: A Discussion
When ascending a steep
gradient in most cars, it is useful to have an understanding of where the
engine offers adequate performance, and so when to change down, or what gear to
tackle the hill in.
This is more important in a
lower powered car since there is less to work with!
I’m going to use our Ka
tackling the A66, Penrith to Scotch Corner. This road features some steep uphill
gradients, many on the dual carriageway.
At 60 mph and in top gear,
the Endura-E is churning over at approximately 2,500
rpm - where the donk produces peak torque.
If we slow down but keep
Kermit in fifth gear, the torque output of the engine is reduced. This reduces the engine’s ability to pull the
Ka up the hill, so what usually happens is that the Ka decelerates.
So if we tackle a steep
enough hill, once the speed gets below 60, it will not accelerate. Changing down to fourth is the best option:
60 in fourth is approximately 3,150 rpm, and peak
torque is delivered at 48 mph.
If the speed drops below
48, third beckons! Peak torque in third
gear is delivered at 35 mph.
The Ka tackles hills in top
gear much more effectively at 70 mph - as the speed drops, so the torque
generated increases.