STP Technical Class: Why Atomise?
Posted on January 23 2019
What’s Special About an Injector?
ASNU performance injectors are designed to provide a highly atomized/uniform spray pattern with a tightly controlled flow rate, good response times and high linearity.
Atomization or the spray pattern provided by the injector for the engine has a large number of effects on how the engine can use the fuel you provide it
Liquid Gasoline is not flammable, Gasoline vapor is though.
A small droplet of fuel has a very large surface area compared to its liquid volume, this large ratio of area to volume allows the liquid to be converted to vapor faster by the engine heat, than with large liquid droplets.
High-performance engines normally rev faster than a production engine this means that the liquid fuel has to be converted to vapor more quickly as there is less time per engine revolution
Why a uniform spray pattern ?– "That’s not how my STD factory injector was".
Your standard factory injector was designed to allow the engine to meet the various exhaust emissions standards when looking for power requirements for the injector change.
What other things affect the combustion of fuel the injector provides?
The process of combustion in an internal combustion engine is complex, the combustion of the air-fuel mixture is not as many people think an explosion but a gradual burn at high speed giving a progressive rise in pressure inside the cylinder as heat and pressure are generated
The flame front is the process, through which the ignition of the air-fuel mix is passed from the point of ignition (spark plug) to the outer areas of the combustion chamber.
How does the Engine turn this burning of the Fuel into Power?
The full combustion of the air-fuel mixture results in an increase in heat in the cylinder, as you increase the heat in the cylinder the air and combustion gasses try to expand but because they are limited by the cylinder volume the pressure rises instead. The pressure curve starts off low and gradually increases up to a peak. This rise in pressure drives the piston down in the cylinder just like pedaling a bike, as the piston moves down in the cylinder the volume available increases and the pressure then drops due in a similar curve due to the nature of the crank rotation even though the mixture is still burning and expanding.
How can I make more Power?
The easiest way for an engine to produce more power is for it to complete the combustion cycle more frequently – make it revs faster, however there is a limit for a fuel droplet size as to how quick you can achieve the vaporization and combustion process and this will limit the engine max speed as much as any mechanical factor
As a rule the bigger the piston diameter the slower the engine can complete the process
What happens if I don’t complete the process?
If you don’t complete the combustion process before the engine gets to the bottom of the piston stroke when the exhaust valve opens you will still have a burning mixture this will heat the exhaust and confuse the Lambda Sensor ( if you have one ) as there is oxygen that is unused at the time it passes the sensor fooling the engine ECU into thinking there is a lean mixture when you already have excess unburnt fuel. At this point, the ECU or the tuner frequently add more fuel because of the lean signal from the Lambda. Adding more fuel normally slows the combustion making the problem worse. This is why you often see cars black smoking (unburnt fuel shows in the exhaust gas as a black soot/smoke) but the tuners are adamant that they are running lean.
Can I advance the timing increase the available burn time?
In short NO. The pressure generated in the cylinder before TDC is actually robbing the engine of power as the engine is using the momentum (rotational power) gained during previous power strokes to overcome this pressure to get to TDC.
OK now for the Detailed bit !!!!
In the curves below the different curves show the different flame front speeds and the effects they have on the pressure rise, all the traces show almost the same cylinder pressures this is what the engine actually converts to HP, the average of this is given as Brake Mean Effective Pressure or BMEP.
The shape you get is a distinctive Bell shape shown below. The time from the start of the rise to the top of the peak is a function of the flame front propagation, the curve after the peak is controlled by the flame front and the piston movement, but is also affected by shrouding and turbulence.
The optimum point for the peak pressure to occur is 15 to 20 degrees after top dead center (ATDC) depending on the engines mechanical layout.
The speed of Flame front propagation is dependant on several variables (some of which will be covered in greater detail later).
- Atomization of fuel in charge air.
- Relative Compression ratio to include
- Compression ratio
- Barometric air pressure
- Boost pressure
- Air temp
- Air fuel ratio.
The Optimal flame front speed (the fastest speed) occurs with a well-atomized fuel allowing faster vaporization of the fuel and a better distribution of the vapor within the cylinder. This is because the fuel molecules are closer together allowing the flame to pass to the next molecule sooner and giving up less exothermic energy in the process.
The closer the molecules are together the faster the flame front can travel to the extremities of the cylinder.
2 Relative cylinder CR
The relative cylinder compression ratio can be split down into 5 main areas of concern they all affect the flame front for the same reason e.g. fuel molecule density and therefore the distance between the fuel molecules in the head chamber
The higher the operating compression ratio the faster the flame front will travel, this is because with an increase in CR you have decreased the distance between the fuel molecules buy forcing them into a fixed volume head chamber.
Barometric Air pressure
This will vary depending on altitude, weather condition, and ambient air temp (this is why dyno figures are corrected for altitude).
For those of you with some practical experience of this, the below link is a nice converter for barometric conditions however there is no correction for temperature so it will require correction for air temperature
The higher above sea level you are the lower the air pressure will be. This means that the air is less dense reducing the amount of fuel you can add to achieve the same air-fuel ratio and therefore there will be fewer fuel molecules within the head chamber – therefore they will be further apart slowing the flame front for a given level of atomization
The chart below shows std heights, temps, and pressures for correction
Below is a useful link for air pressure at any given altitude and the amount of oxygen present as a percentage vs. sea level
The STD for air pressure at sea level is 1013 MBA at sea level however weather will vary this down as far as 970 MBA typically poor weather, rain, and high winds. For clear skies and good weather, the pressure can rise up as far as 1030mba. It is useful to have the real barometric figure when mapping engines so you have a base and can back calculate the worst case AFR ratio. Weather also can have a secondary effect through moisture reducing the air inlet and combustion temperatures.
Ambient air temperature
Temperature plays a large part in air density and the typical temp at sea level of 15degrees would equate to a temperature of -15 at 14000 ft (the top of the Pikes Peak hill climb) the lower the temperature the denser the air will be (this is why turbo cars use intercoolers) and therefore more fuel can be added and the closer the fuel molecules will be in the head chamber and the faster the flame front will move
And here is a link so you don’t have to work out pressure, temp to density
Air Fuel Ratio
Air fuel ratio is commonly known as AFR also affects burn speed the actual theoretical ideal AFR for an internal combustion engine running on gasoline is 14.7 to 1
That is 14.7 kg of air for every 1kg of gasoline this is known as the stoichiometric point, any more gasoline than this and its just going out the exhaust as there is not enough oxygen to burn the excess gasoline. The different fuels all have different stoichiometric points and this must be considered if a different fuel is to be used,
Methanol is 6.47 to 1, and Ethanol is 9 to 1.
See the link below for a full chemical explanation
Below is an example calculation of AFR at 14.1 to 1 for gasoline
How much Air?
1.204kg per 1000liters of air at sea level and 20celcius and using the STD calculation for air density gives the 14.7 ratios of air gives14.7/ 1.204kg = 12.209 x 1000 = 12209 liters of air.
How much Fuel?
Because gasoline has an approximate density of 0.75 kg/l (it depends exactly on the blend and the brand) for the 1 kg of fuel we require 1.36liters of gasoline
So at a 14.7 to 1 ratio at sea level and 20Celcuis
12209 liters of air to 1.36 liters of gasoline
Or 12209/1.36 =8977 litres
So 8977 liters of air requires 1 liter of gasoline at sea level and at 20celcuis
However, due to the pure mechanics elements, the best actual operating AFR for an internal combustion engine to achieve the best flame growth (therefore best mean cylinder pressure or BMEP rise) is actually between 12.4 and 13 to 1 depending on the mechanical design of the engine
If the mixture is any richer (numbers lower than 12.3 to 1) or leaner (numbers higher than 13.1 to 1) then the flame propagation across the head chamber will be delayed.
Ignition timing and its relationship with flame front propagation.
There is a commonly know relationship between ignition advance, AFR, and Boost. This, however, is not really the truth, what is being seen is the effect of Flame front propagation
The flame front as previously stated in the other articles always travels at a fixed speed for a given set of conditions, the one thing that has no effect is the engine speed apart from the secondary effects due to cam timing /scavenging cylinder filling improvements at higher engine speed that increases the compression ratio and typically results in better atomization due to higher intake runner airspeeds
To create power from the burning of fuel the engine converts this linear motion of the piston to rotary by means of the crank. The most efficient point for peak cylinder pressure to occur is at 15 to 20 degrees after top dead center (ATDC). To ensure that peak pressure occurs in this window with changing engine speed the ignition timing must be varied
For example, if you were to assume time from ignition to the peak pressure occurring for a naturally aspirated gasoline engine would be 2.5 ms on an engine running without a throttle (standardized manifold vacuum)
At 800 rpm a typical idle speed the time period from TDC to 15 is
The calculation for this is as follows
850 rpm / 60 = 14.166 revolutions per second
14,166revs X 360 = 5099.76 degrees per second
1 second / 5099.76 degrees = 0.000196087 seconds per degree
0.000196087 s/degree x 15 degrees = 0.00291305 seconds
0.00291305seconds X 1000 = 2.941ms
In this case, the point of ignition required for optimum timing would actually be after TDC. The exact point of ignition can be calculated by calculating the difference in degrees between 2.5ms and 2.94ms.
The calculation for this is as follows
2.94ms – 2.5ms = 0.44ms
As calculated earlier there is 0.196ms/degree at 800 rpm
0.44ms / 0.196 ms/degree = 2.24 degrees
This would mean that the ignition timing would be set to 2.24 degrees ATDC.
At 8000 rpm at the same time to reach peak pressure, the timing would have to be advanced
So as before
8000 rpm / 60 = 133.33 revolutions per second
133.33 revs X 360 = 47998.8 degrees per second
1 second / 47998.8 degrees = 0.00002083 seconds per degree or 0.02ms/degree
So to get the ignition advance required for a 2.5ms burn would give 110 degrees of ignition advance before top dead center (BTDC) if all factors stayed the same
So the importance of ignition timing for the best positioning of the peak pressure to the rotation of the engine can be seen from this set of calculations.
However normal levels of ignition advance are in the range of 10 degrees BTDC at idle to 40 degrees BTDC at full load so how can this happen when we consider our theoretical 110 degrees BTDC?
The difference between the theoretical single conditions calculated above is due to speed and load based factors
The corrections required are for the following conditions
- Atomization improvements due to higher airspeed resulting fuel staying in suspension in the air stream.
- throttle position – Manifold vacuum at idle vs. Wide Open Throttle ( WOT)
- Passive pressure charging (the function of Cam profile/overlap)
- Ambient air temp in the cylinder ( less heat pick up due to increased speed)
So why is all this important?
If the ignition point is optimal for the peak pressure to occur at the right time but you have a slow flame front due to poor atomization or over rich/lean AFR then you will have to ignite the air-fuel mixture earlier BTDC.
Any pressure that is generated in the cylinder before TDC is actually robbing the engine of power as the engine is using the momentum (rotational power) gained during previous power strokes to overcome this pressure to get to TDC.
The area under this curve is effectively HP that you are loosing, therefore, a faster burn will result in a higher output HP if all other factors remain constant as there is a smaller power loss during the compression stroke. Therefore the smaller ignition advance resulting from a faster flame propagation incur fewer compression losses, in addition to this the faster the engine revs the more savings you will realize.
Any small reduction in the compression losses at 8000 rpm will result in a significant increase and these improvements can in certain circumstances be between 2 and 5 % reductions in losses
As an example, a 2000cc engine producing 500hp at 8000rpm with a 0.5% reduction in pumping losses would experience a 2.5 hp increase
500/4000=0.125 hp per rev
0.125 / 4 = 0.03125 hp per cylinder per rev
0.03125 X 1.005= 0.03140 hp per cylinder per rev
0.03140 X 4 X 4000 = 502.5
There is also a commonly held belief that more ignition advance will produce more power. To achieve this AFR is increased slowing the flame front down
There is also a commonly held belief that more ignition advance will produce more power. To achieve this many tuners increase the AFR slowing the flame front speed down to compensate for an over-advanced ignition point – effectively over fuelling to slow the burn to a preset ignition point.
We at STP hope that this Technical Blog can help solve any issues you may be facing. Stay tuned for more Technical Blogs.
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