Relentless
Well Known Member
This question comes up during most calls and I know the concept is not mainstream yet so here is an attempt to start conversation about our new ACE AX50 cylinders and the unleaded fuel advantage of our components. While they will run leaded fuel just fine, we would actually prefer our customers ran unleaded fuel exclusively.
Cleaner engine, better economy, and many synthetic oil options are some real reasons for looking at the R Series components.
How is it possible to use unleaded, lower octane fuel with R Series engines developed by ACE Performance and now available from Titan Engines?
First, we need to understand a little about fuel octane ratings, there are a lot
of books on the subject but basically the higher the octane rating the more resilient to the fuel is to knock (pre-ignition).
Commercial fuel blends have a scale that runs from 0~100 and these
numbers are based on two fundamental hydrocarbons, one knock resilient
(100)the other exceedingly susceptible to knock (0), this is basically
how the two ends of the spectrum are decided.
Knock rating of a fuel is determined by running a test engine under
strictly controlled conditions. The octane number achieved depends on
the type of controlled testing. Currently there are two fundamental
methods, one is RON and the other is MON. This controlled testing though
does not really correlate with real world operating conditions. It has
been found to be somewhere in between the two testing methods and
therefore and average of the two is taken RON+MON/2
Contrary to popular belief, putting in 100 octane fuel will not give you
an instantaneous 20hp gain. It will however, as an example, *allow you to
achieve the higher hp figure by using a higher compression piston.
Secondly and more importantly is the design of the combustion chamber
and the effects it will have on an octane number.
As we increase the compression ratio, the cylinder pressure and
temperature will rise to such a level that eventually the end gas will
spontaneously burn. This is the reason why a higher octane fuel is used
to suppress this spontaneous burn (detonation) with increasing
compression ratio.
Here is a rough Minimum octane necessary with specific compression ratios for standard Lycoming engines:
Octane/Compression
50/5.0
70/6.8
80/8.0
90/8.7
100/9.0+
The detonation itself, which is a function of the end gas spontaneously
burning, will have shock-waves moving through the cylinder charge at
velocities in the region of 1200m/s. These waves reflect against the
cylinder and combustion chamber wall, these are then set into a state of
vibration which you hear as a ping or a knocking sound.
There are many methods, but the following or a combination will
considerably reduce the onset of knock.
1.) Increase the charge turbulence and flame travel distance, this reduces the
mixture end temperature.
2.) Reduce the flame path time this allows for a reduction in ignition
timing event.(If peak pressure occurs at TDC then higher pressures and
temperatures will occur)
3.) Ensure design of the mechanical components are not conducive to knock.
4.)Raise the octane number.
The AX50 cylinder and pistons are designed as an optimized package that
takes careful consideration into account of the above.
The combustion chamber is radical departure from the standard 360 type
cylinder head. It has a recessed chamber with squish bands (the charge
is squeezed radially inwards towards the center of the combustion
chamber),this promote higher than average flame speed along with
induced swirl by this means we are able to mix vigorously the fuel mixture before
combustion which will assist in a faster combustion pressure rise,
allowing for a reduction in ignition timing and at the same time
allowing for a considerably leaner mixture.
(image of the surface of a AX50 piston burn pattern)
The spark plug location on the AX50, whilst similar from the outside is
again very different on the inside, the plugs are much closer together
allowing a substantial reduction in timing due to the increase in burn
speed.
(AX50 Cylinder showing close location of spark holes as well as bathtub shape of combustion chamber and squish band)
The AX50 piston, has a shape that mimics the cylinder combustion shape
and this shape forces the charge into the center. The top land on the
AX50 piston is greatly reduced and this minimizes the crevice volume.
The crevice volume is the area around the top of the piston down to the
top of the top ring, this fills with unburnt fuel and if
temperatures increase too much, the unburnt fuel spontaneously ignites.
(AX50 piston shape and small upper land)

So, why can we run lower octane unleaded fuel?
www.ACE-Performance.com totally redesigned the cylinder and piston assembly
incorporating known, scientifically proven design details into our
product that the standard 360 and (soon 320) type engines simply do not have.
Cleaner engine, better economy, and many synthetic oil options are some real reasons for looking at the R Series components.
How is it possible to use unleaded, lower octane fuel with R Series engines developed by ACE Performance and now available from Titan Engines?
First, we need to understand a little about fuel octane ratings, there are a lot
of books on the subject but basically the higher the octane rating the more resilient to the fuel is to knock (pre-ignition).
Commercial fuel blends have a scale that runs from 0~100 and these
numbers are based on two fundamental hydrocarbons, one knock resilient
(100)the other exceedingly susceptible to knock (0), this is basically
how the two ends of the spectrum are decided.
Knock rating of a fuel is determined by running a test engine under
strictly controlled conditions. The octane number achieved depends on
the type of controlled testing. Currently there are two fundamental
methods, one is RON and the other is MON. This controlled testing though
does not really correlate with real world operating conditions. It has
been found to be somewhere in between the two testing methods and
therefore and average of the two is taken RON+MON/2
Contrary to popular belief, putting in 100 octane fuel will not give you
an instantaneous 20hp gain. It will however, as an example, *allow you to
achieve the higher hp figure by using a higher compression piston.
Secondly and more importantly is the design of the combustion chamber
and the effects it will have on an octane number.
As we increase the compression ratio, the cylinder pressure and
temperature will rise to such a level that eventually the end gas will
spontaneously burn. This is the reason why a higher octane fuel is used
to suppress this spontaneous burn (detonation) with increasing
compression ratio.
Here is a rough Minimum octane necessary with specific compression ratios for standard Lycoming engines:
Octane/Compression
50/5.0
70/6.8
80/8.0
90/8.7
100/9.0+
The detonation itself, which is a function of the end gas spontaneously
burning, will have shock-waves moving through the cylinder charge at
velocities in the region of 1200m/s. These waves reflect against the
cylinder and combustion chamber wall, these are then set into a state of
vibration which you hear as a ping or a knocking sound.
There are many methods, but the following or a combination will
considerably reduce the onset of knock.
1.) Increase the charge turbulence and flame travel distance, this reduces the
mixture end temperature.
2.) Reduce the flame path time this allows for a reduction in ignition
timing event.(If peak pressure occurs at TDC then higher pressures and
temperatures will occur)
3.) Ensure design of the mechanical components are not conducive to knock.
4.)Raise the octane number.
The AX50 cylinder and pistons are designed as an optimized package that
takes careful consideration into account of the above.
The combustion chamber is radical departure from the standard 360 type
cylinder head. It has a recessed chamber with squish bands (the charge
is squeezed radially inwards towards the center of the combustion
chamber),this promote higher than average flame speed along with
induced swirl by this means we are able to mix vigorously the fuel mixture before
combustion which will assist in a faster combustion pressure rise,
allowing for a reduction in ignition timing and at the same time
allowing for a considerably leaner mixture.
(image of the surface of a AX50 piston burn pattern)
The spark plug location on the AX50, whilst similar from the outside is
again very different on the inside, the plugs are much closer together
allowing a substantial reduction in timing due to the increase in burn
speed.
(AX50 Cylinder showing close location of spark holes as well as bathtub shape of combustion chamber and squish band)
The AX50 piston, has a shape that mimics the cylinder combustion shape
and this shape forces the charge into the center. The top land on the
AX50 piston is greatly reduced and this minimizes the crevice volume.
The crevice volume is the area around the top of the piston down to the
top of the top ring, this fills with unburnt fuel and if
temperatures increase too much, the unburnt fuel spontaneously ignites.
(AX50 piston shape and small upper land)

So, why can we run lower octane unleaded fuel?
www.ACE-Performance.com totally redesigned the cylinder and piston assembly
incorporating known, scientifically proven design details into our
product that the standard 360 and (soon 320) type engines simply do not have.
