What's Churning in your Engine ? (Click here to Print this page)
This column is for petrol-heads. For those who don’t know
what that means: they prefer to go to bed with a V8 instead of Julia Roberts.
They like the sound of cammed engine to von Karajan
conducting the Berlin Philharmonic. They will go ecstatic by smelling
the exhaust gas compared to Nina Ricci that their girls would be wearing. In
case this sounds lunatic to you, you are right. You are not in the right place
baby!
This being the first one of hopefully some
interesting dope for all you octane lovers, let me try to get your hands dirty.
If after you read this stuff you don’t feel excited to try out some of the
stuff yourself or at least down a few pints of heated discussions with fellow
gas-heads, then this column is a waster.
OK so here we go. The first column is pure gas! Get your fundamentals on gas right before you try out anything. So what’s your gas? You need to get the Octane right. So…
Octane.
What parameters determine octane
requirement?
What is the Octane Number Requirement of your Vehicle?
First the basic “funda”. The
actual octane requirement of a vehicle is called the Octane Number Requirement (ONR), and is determined by using series of standard octane
fuels that can be blends of iso-octane and normal heptane ( primary reference
), or commercial gasolines ( full-boiling reference ). The vehicle is tested
under a wide range of conditions and loads, using decreasing octane fuels from
each series until trace knock is detected. The conditions that require maximum
octane are not consistent, but often are full-throttle acceleration from low
starting speeds using the highest gear available. They can even be at constant
speed conditions, which are usually performed on chassis dynamometers. Engine
management systems that adjust the octane requirement may also reduce the power
output on low octane fuel, resulting in increased fuel consumption, and
adaptive learning systems have to be preconditioned prior to testing. The
maximum ONR is of most interest, as that usually defines the recommended fuel,
however it is recognised that the general public seldom drive as
severely as petrol-heads, and so may be
satisfied by a lower octane fuel.
What is the effect of Compression ratio?
Most of you know that an increase
in Compression Ratio will require an increase in fuel octane for the
same engine design. Increasing the compression ratio increases the
theoretical thermodynamic efficiency of an
engine according to the standard
equation
Efficiency = 1 - (1/compression
ratio)^gamma-1
where gamma = ratio of specific
heats at constant pressure and constant volume of the working fluid ( for
most purposes air is the working fluid, and is treated as an ideal gas ).
There are indications that thermal efficiency reaches a maximum at a
compression ratio of about 17:1 for gasoline fuels in an Spark
Ignition engine. (Ha! Don’t say “phew”, any
basic text book on thermodynamics
of IC engine will give you this!!!)
The efficiency gains are best
when the engine is at incipient knock, that's why knock sensors ( actually
vibration sensors ) are used. Low compression ratio engines are less efficient
because they can not deliver as much of the ideal combustion power to the
flywheel. For a typical carburetted engine, without engine management :
Compression Octane
Number Brake Thermal Efficiency
Ratio
Requirement ( Full
Throttle )
5:1
72 -
6:1
81 25 %
7:1
83 28 %
8:1
87 30 %
9:1
91 32 %
10:1
93 33 %
11:1
97 34 %
12:1 100 35 %
Modern engines have improved
significantly on this, and the changing fuel specifications and engine design
should see more improvements
What is the effect of changing the air-fuel ratio?
Modern
engines, with engine management systems, now have their maximum octane
requirement near to 14.5:1. For a
given engine using gasoline, the
relationship between thermal efficiency, air-fuel ratio, and power is
complex. Stoichiometric combustion ( air-fuelratio = 14.7:1 for a typical
non-oxygenated gasoline ) is neither maximum power - which occurs around
air-fuel 12-13:1 (Rich), nor maximum thermalefficiency - which occurs around
air-fuel 16-18:1 (Lean). The air-fuel ratio
is controlled at part throttle by
a closed loop system using the oxygen sensor in the exhaust. Conventionally,
enrichment for maximum power air-fuel ratio is used during full throttle
operation to reduce knocking while providing better drive-ability. An average
increase of 2 (R+M)/2 ON is required for each 1.0 increase (leaning)
of the air-fuel ratio. If the mixture
is weakened, the flame speed is
reduced, consequently less heat is converted to mechanical energy, leaving
heat in the cylinder walls and head, potentially inducing knock. It is
possible to weaken the mixture sufficiently so
that the flame is still present
when the inlet valve opens again, resulting in backfiring.
What is the effect of changing the ignition timing?
The tendency to knock increases
as spark advance is increased. For an engine with recommended 6 degrees BTDC (
Before Top Dead Center ) timing and 93 octane fuel, retarding the spark
4 degrees lowers the octane requirement to 91, whereas advancing it 8
degrees, requires 96 octane fuel. It
should be noted this requirement depends
on engine design. If you advance the spark, the flame front starts earlier,
and the end gases start forming earlier in the cycle, providing more time
for the auto-igniting species to form before the piston reaches the optimum
position for power delivery, as determined by
the normal flame front
propagation. It becomes a race between the flame front and decomposition of the
increasingly-squashed end gases. High-octane fuels produce end gases that take
longer to auto-ignite, so the good flame front
reaches and consumes them
properly.
The ignition advance map is
partly determined by the fuel the engine is intended to use. The timing of
the spark is advanced sufficiently to ensure that the fuel-air mixture burns
in such a way that maximum pressure of the burning charge is about 15-20
degree after TDC. Knock will occur before this point, usually in the late
compression - early power stroke period.
The engine management system uses
ignition timing as one of the major variables that is adjusted if
knock is detected. If very low octane fuels are used ( several octane numbers
below the vehicle's requirement at optimal settings ), both performance and
fuel economy will decrease.
The actual ignition timing to
achieve the maximum pressure from normal combustion of gasoline will
depend mainly on the speed of the engine and the flame propagation rates in the
engine. Knock increases the rate of the pressure rise, thus superimposing
additional pressure on the normal combustion pressure rise.
Here is something that might
interest you. One petrol-head of my vintage, Somender
Singh has done some phenomenal work on combustion chamber design
that changes the way flame propagates inside the combustion chamber. This
allows him to use very much higher ignition advance. This design has been
awarded a US patent. You can write to him at the following address : garudarad@eth.net
What is the effect of engine management systems?
Engine management systems are now
an important part of the strategy to reduce automotive pollution. The
good news for you is their ability to maintain the efficiency of
gasoline combustion, thus improving fuel
economy. The bad news is their tendency to hinder tuning for power. A very basic modern engine system could monitor and control:- mass air flow, fuel flow, ignition timing, exhaust oxygen ( lambda oxygen sensor ), knock ( vibration sensor ), EGR, exhaust gas temperature, coolant temperature, and intake air temperature. The knock sensor can be either a non resonant type installed in the engine block and capable of measuring a wide range of knock vibrations ( 5-15 kHz ) with minimal change in frequency, or a resonant type that has excellent signal-to-noise ratio between 1000 and 5000 rpm.
A modern engine management system
can compensate for altitude, ambient air temperature, and fuel octane. The
management system will also control cold start settings, and other
operational parameters. There is a new requirement
that the engine management system
also contain an on-board diagnostic function that warns of
malfunctions such as engine misfire, exhaust catalyst failure, and evaporative
emissions failure. The use of fuels with alcohol such as methanol can confuse the
engine management system as they generate more hydrogen which can fool the
oxygen sensor.
The use of fuel of too low octane
can actually result in both a loss of fuel economy and power, as the
management system may have to move the engine settings to a less efficient part
of the performance map. The system retards
the ignition timing until only
trace knock is detected, as engine damage from knock is of more consequence
than power and fuel economy.
What is the effect of temperature and load?
Increasing the engine
temperature, particularly the air-fuel charge temperature, increases the
tendency to knock. The Sensitivity of a fuel can indicate how it is affected by
charge temperature variations. Increasing load increases both the engine
temperature, and the end-gas pressure, thus the likelihood of knock increases
as load increases. Increasing the water
jacket temperature from 71C to
82C, increases the (R+M)/2 ONR by two.
What is the effect of engine
speed?.
Faster engine speed means there
is less time for the pre-flame reactions in the end gases to occur, thus
reducing the tendency to knock. On engines with management systems, the
ignition timing may be advanced with engine
speed and load, to obtain optimum
efficiency at incipient knock. In such cases, both high and low engines
speeds may be critical.
What is the effect of engine deposits?
A new engine may only require a
fuel of 6-9 octane numbers lower than the same engine after 25,000 km. This
Octane Requirement Increase (ORI) is due to the formation of a mixture of
organic and inorganic deposits resulting from
both the fuel and the lubricant.
They reach an equilibrium amount because of flaking, however dramatic
changes in driving styles can also result in dramatic changes of the
equilibrium position. When the engine starts to burn
more oil, the octane requirement
can increase again. ORIs up to 12 are not uncommon, depending on driving
style . The deposits produce the ORI by several mechanisms:-
- they reduce the combustion chamber volume, effectively
increasing the compression ratio.
- they also reduce thermal conductivity, thus increasing the
combustion chamber temperatures.
- they catalyze undesirable pre-flame reactions that produce end
gases with low auto-ignition temperatures.
What is the effect of air temperature?
An increase in ambient air
temperature of 5.6C increases the octane requirement of an engine by 0.44
- 0.54 . When the combined effects of air temperature and humidity
are considered, it is often possible to use one octane grade in summer, and
use a lower octane rating in winter.
What is the effect of altitude?
The effect of increasing altitude
may be nonlinear. The gurus say that a decrease of the octane
requirement of 1.4 RON/300m from sea level to 1800m and 2.5 RON/300m from 1800m to
3600m.
The larger reduction may be
required in old engines because:-
- reduced air density provides lower combustion temperature and
pressure.
- fuel is metered according to air volume, consequently as density
decreases the stoichiometry moves to rich, with a lower octane number
requirement.
- manifold vacuum controlled spark advance, and reduced manifold
vacuum results in less spark advance.
What is the effect of humidity?
An increase of absolute humidity
of 1.0 g water/kg of dry air lowers the octane requirement of an engine
by 0.25 - 0.32 MON .
Now you have some basic
understanding of what happens to fuel inside your engine.
This is the right time in Kolkata
to check this out. It is hot and humid and roads will be chocked with traffic.
That means engines will be really fuming hot. The moment you get a clean
stretch of road, shift to the highest gear possible and slam the throttle down.
If you can hear that familiar “pinging” noise, you should be able to figure out
what to do.