By: Haitham Alhumsi
The two main threats to a performance engine’s longevity are pre-ignition and detonation. Our horsepower calculator now includes an Octane Calculator and a preignition combustion chamber temperature model… these two new modules that work hand in hand at making sure your desired build up is smart and safe by fending off the probability of pre-ignition.
As far as detonation is concerned, the power calculator already helps you ensure a safe and controlled combustion process by avoiding the primary causes of detonation:
1- Too much timing advance (using the tuning tab’s recommended timing curve)
2- Not enough fuel for your air flow level or for your volumetric efficiency (the power calculator gives you adequately sized recommendations for fuel pumps, lines, and injectors)
Going back to the topic of preignition… The two new features in our calculator are our new Octane Center and our new Temperature Model.
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Our Octane Center allows you to mix your own home-brew race fuel starting with a basis of well known hydrocarbon fuels.
This table includes:
- Oxygenated and non-oxygenated fuels.
- Nitrogen enriched fuels and regular fuels
- Alocoholic and non-alcoholic fuels
Our octane center allows you to mix fuels by volume (as you would typically measure out fuel by the gallon, liter or quart…etc) and then gives you the approximate characteristics of the resultant mixture including:
- The mix’s ‘advertised’ octane number
- The mix’s final energy density per liter
- The mix’s stoichiometric and best power air to fuel ratios (calculated by weight as they should be)
- The mix’s approximate auto-ignition-temperature
- The mix’s simple power factor (compared to gasoline when performing a direct fuel conversion – for example converting from gasoline to E85 without touching the engine setup).
- The mix’s potential power factors (both boosted and naturally aspirate) when converting from gasoline to the new mixture AND taking full advantage of the new mixture’s octane rating and auto-ignition properties by pushing the boost level or compression ratio up to their new limits.
- The mix’s exhaust gas factor… different fuels produce different volumes of exhaust gas for the same horsepower rating due to both the fuel’s energy density and the molecular byproducts of combustion.
Knowing your fuel’s attributes now changes EVERYTHING in the power calculator:
- Intake to exhaust ratios change for different fuel exhaust gas factors.
- Intake cubic feet per minute for a horsepower target changes based on a fuel’s energy density which changes the intake system sizing requirements.
- Your fuel supply requirements and the sizes of your pump, feed and injectors change based on the new fuel’s energy density, stoichiometry, and best power lambda
- Your maximum safe boost pressure and compression ratio change based on the new fuels’ auto-ignition (preignition) temperature
- Last but not least any choice of turbo or supercharger sizing changes because:
- Doing the fuel conversion changes your stock engine’s power level which affects how much boost you need to run to reach your power goals.
- The new fuel’s energy density changes the target flow (in CFM) required by your turbo to reach your power goal.
…. so choosing your turbo or supercharger now comes at a different pressure ratio and target CFM compared to gasoline and that changes everything.
More importantly … the octane center not only affects our calculations, but it also interfaces with the new temperature model.
Knowing the details of your setup including:
- Charger Efficiency
- Intercooler Efficiency
- Presence of Water/Alcohol injection
- Compression ratio
- Ambient intake temperature
We can now calculate your final in cylinder combustion chamber temperature and compare it to the fuel mixture’s approximate preignition temperature which gives us an insightful calculation for the probability of preignition for a specific combination of compression, boost, efficiency and charge cooling.
There are two ways to look at preignition:
1- Theoretically speaking, the auto ignition temperature of a fuel (or a compound) is not a strict temperature but rather a complex probability curve. Every material, has a temperature beyond which the amount of heat stored or consumed by the material exceeds the amount of heat radiated or released by it. Once this point is reached, the material slowly and gradually stores more and more energy until it reaches its point of spontaneous combustion or auto-ignition without the need for an external spark to catalyze the burn process.
When this happens in an engine the mixture explodes pre-maturely (before the spark plug fires) while the piston is still early in the compression stroke, which causes even more thermal rise and a runaway chain reaction of preignition that rapidly destroys the engine at its weakest link (be it the piston crown, the ring land, the valve stem, or the connecting rod depending on the specific build up)…
Because this is a timed process, strictly speaking, scientific literature no longer talks about a singular auto ignition temperature but rather a an auto ignition probability curve that increases in probability as the temperature increases as well as an auto ignition time delay which describe how long it takes a fuel to go from being neutral to spontaneous combustion when starting at a specific ambient temperature.
For practical applications, in an engine where the air fuel mixture is actively heated and compressed and trapped (thermally) inside a hot combustion chamber with a hot spark source of ionized air (in the spark plug) and 900*C exhaust gas possibly reverting back in with exhaust gas recirculation then using the minimum temperature on the auto ignition curve is a good conservative estimate of a temperature that you never want to reach with your air and fuel mixture if you want to keep your engine safe from preignition.
Similarly, the scientific standard of observing a heated mixture for at least 5 seconds before making a call on weather or not the temperature has a reasonable probability or preignition doesn’t mean much compared to a 13 second ¼ mile run where the mixture is heated and abused at full throttle for 13 seconds straight or during a top gear pull where you are in 5th gear at full boost and power on the back straight of your favourite racing circuit.
2- From a practical point of view, there are many real world factors that affect thermal management in the combustion chamber including:
- Cylinder wall cooling with open deck vs closed deck blocks
- Cylinder head cooling with cast iron heads vs aluminum heads as a good example
- Spark plug heat range choice and the ability of the spark plug to release heat into the head rather than act as a hot spot in the chamber
- Piston surface temperatures which is affected by oil squirters under the piston, piston top thermal coatings (that help distribute the heat more evenly around the piston crown to prevent isolated hot spots, but has the negative side effect of reflecting heat energy back into the combustion chamber which can raise the efficiency of a naturally aspirated car but can also push a high boost application over its preignition limits).
- Exact camshaft timing & the exhaust system back-pressure which can dramatically affect the dynamic compression ratio (and thus the temperature rise due to the mechanical compression of the air and fuel mixture) as well as the amount of exhaust gas reversion and intake air dilution.
However, these are all minor contributors when compared to the choice of using gasoline with an auto-ignition temperature of around 280*C with an Alocoholic fuel with an auto-ignition temperature of 410*C …
Using the fuel mixture’s calculated preignition temperature limit we give you a dynamic model that allows you to test different combinations of boost and compression and charge cooling, find your final density ratio for the mixture, find your real world expected horsepower using that combination of fuel, compression, boost, and cooling, and find your probability of preignition for that setup…
In a few minutes (and without any complex equations or mathematics) we give you a model that allows you to maximize the utilization of your current setup, while still being smart and safe… and although that isn’t a 100% accurate model, it still is leaps and bounds better than just winging it and hoping that things remain safe…
The same equations used by the racers in Rally (Toluene), Formula (Ethanol), and Top Fuel drag (Nitromethane)… delivered to you through an easy to use graphical interface … to maximize your buildup, without having to learn all of the equations…
Here is an example of 4 different setups shooting for 900hp on an LS1 V8 engine. As you can see the four different fuels (listed in the first row) dramatically change the specifications of the buildup and the ultimate power potential of the build…