BE A MAN!Take control of your engine… Use you MegaSquirt!
Take Control! • …and you want to make it run…fast! • Lets say you have this engine….. …but how do you control it?
Take Control! • You will need the following three things: Air Fuel Ignition
Take Control! • And you have to mix the Air and the Fuel in just the right amount for best results. • Its like making the perfect Gin and Tonic: Recipe: 3 Ounces Gin 4 Ounces Tonic Note that Al Grippo prefers more of the Hydrocarbon (Gin – CH3CH2OH) than Diluent (Tonic)… he’s just that way….
Take Control – Its in the Ratios! • For fuel and air to form combustion there needs to be specific ratios of each. • For a given Hydrocarbon (that’s gasoline) there needs to be a proper amount of oxygen (in the air). • The most efficient form of combustion is when the ratios are in Stoichiometric proportions… all this means is that all of the hydrocarbon reacts with all of the oxygen, producing water and carbon dioxide. • If you have too much air and not enough fuel, there will be excess oxygen (lean) • If you have too much fuel and not enough oxygen then there will be leftover hydrocarbons, CO, H2, etc. (rich).
Take Control – Simpler is Better! • The AIR-FUEL ratio is just the ratio of the air to the fuel: • Another way to quantify is with Lambda: • Still another way is the Equivalence Ratio (my favorite):
Take Control of Your Hydrocarbon • OK – lets start with the fuel, this one is the easy parameter to control! • An electromechanical valve known as a Fuel Injector does the work for you… • If you apply 12 volts to the terminals the internal valve (pintle) opens and lets fuel flow. • Take away the 12 volts and the valve shuts down tight… no fuel flows. • Extremely simple device!
Take Control of Fuel • The fuel flow thru the injector is governed by the following mechanical arrangement:
Mass Rate Of Fuel Injector Coefficient Fuel Density Pressure Differential Across Injector Take Control – Equations are cool! • The fuel flow thru the injector is governed by the following equation:
Take Control – Simpler is Better! • Since we are dealing with one fuel type and one injector, and since the fuel regulator keeps the pressure differential the same for all operating conditions, the previous equation can be reduced to: Fuel_Flow = Inj_Flow_Rate * Pulsewidth • This means that the amount of fuel that flows depends on the Injector Flow Rate and the Open Pulsewidth… its really simple • Note that for small pulsewidths the fuel injector open time starts to dominate and fuel flow is no longer linear – so keep out of this region (this means you)!
All About AIR • OK, we know all about fuel and how it is controlled. It’s the easy part… • Determining the amount of air the engine is sucking in is much, much harder. • Airflow characteristics change all of the time due to: • RPM • Temperature • Pressure • Composition • You gotta know how much air entered the engine in order to match up the proper amount of fuel…..
Mass Air Estimation • So, how does the fuel controller determine the amount of air that the engine breathes? • In other words, we need to know the Mass of the Air flowing into the cylinders. • There are three predominant methods used to do this: • Speed Density (SD) – uses air density, volume, and engine speed to infer the mass air. • Mass-Airflow Meter (MAF) – measures airflow directly with dedicated sensor. • Alpha-N (A/N) – estimates mass air from throttle position and engine speed. • No one method is perfect – we will address each method, their strengths and weakness.
Mass Air Estimation - SD • Speed density infers (or makes an educated estimation) on the amount of air that the engine ingests for each cylinder. • It uses the following facts: • Air has MASS – a “tin can”-full of air actually weighs something. How much it weights depends on how much volume you have trapped and its density. • Air mass within a volume (like the tin can) can be estimated if you know the pressure, volume, and temperature – this is called the Ideal Gas Law (its not just the law… its ideal!) • The engine cylinder, when its at the bottom of the stroke, is just like an empty tin can – it has a volume. • At certain conditions, the intake manifold pressure is darn-near the same as the cylinder pressure, and cylinder is max volume. Good place to sample….
Volume of Tin Can Vd = 3.14159*(bore/2)^2*Stroke (It’s also the displacement divided by the number of cylinders…)
How do you know how much the air weighs (mass) in the tin can? What if the can was filled with ball bearings? Think of air molecules as a bunch of ball bearings... If we know how much a ball weighs and the number of balls we can figure it out!
Lots of ball bearings in a volume is Dense Fewer ball bearings in a volume is Less Dense…
Mass of air molecule is given by its molecular weight. • Molecular mass is one molecule of air relative to the unified atomic mass u which is equal to 1/12 of the mass of one atom of Carbon-12 • Mass of Air = 28.964 g/mole Don’t worry about the “Mole” word above, this is just a unit representing the amount of something – just like the word “Dozen” means “12 of” something….. Avagadro’s number = 6.022 x 1023
Mass Air Estimation - SD • Density is equal to the number of molecules divided by the volume: Number of Molecules Density Volume • Great! We know the volume V of the cylinder… but how do we know the number of air molecules n that are inside? • There is an ideal way to find out!
Mass Air Estimation - SD • The Ideal Gas Law: Universal Gas Constant Pressure Temperature Number Of Moles Volume • This says that the pressure in the cylinder multiplied by the cylinder volume equals the number of molecules of air multiplied by a constant and by the temperature • We can re-arrange the equation and solve for the number of moles “n”……
Mass Air Estimation - SD • If you pick the right kind of R constant (Specific Gas Constant), then this equation is the mass of air at a given pressure, volume, and temperature. • R is .287 kJ/(kg*K) for dry air • P is Pressure in KPa • T is Temperature in deg Kelvin = Deg C + 273
Mass Air Estimation - SD • Lets use variables that represent what we measure on a real engine: Pressure in Manifold Volume of Cylinder At Bottom of Stroke Temperature of Intake Manifold Mass of Air in Cylinder Gas Constant
Mass Air Estimation - SD • We infer the pressure of the cylinder from measuring the pressure in the manifold. The best point to measure this is at the point where the intake valve just closes. Here’s why: • The piston is at the bottom of its stroke. At this point the volume is maximum and is the trapped volume when the intake valve closes and seals off the chamber (a closed tin can). This trapped volume of mass air is what we are trying to match up with mass fuel. • At BTC the piston moves the slowest in its stroke compared to crankshaft motion. This means the volume is not changing as much. So gas flows slow down (and even changes direction back and forth) to the point where the pressure in the manifold (MAP) is very close to the pressure in the cylinder - within 1KPa from tests done at M.I.T. • It is important to sample the MAP at the same point in the engine cycle.
MAP Sample Region Data from MS Forum user MYK777 under SynchroMAP thread. Single cylinder, MAP pressure curve vs. crankshaft
Mass Air Estimation - SD • But – there is a monkey wrench thrown at us in all of this… its call Volumetric Efficiency, or VE. What is VE?? What the &*^% is this?
Mass of gas that actually gets in VE = Mass of gas the cylinder is capable of holding Mass Air Estimation - VE • The piston in the cylinder acts like a volumetric pump – i.e. the piston causes a volume change and hence a corresponding pressure change. • Due to various factors, the amount of fresh air that enters the cylinder is not what the cylinder is capable to actually suck in (i.e. gas transfer due to volume change). • In simple terms, VE is: Denominator comes from the air density based on manifold pressure and temperature
Mass Air Estimation - VE • The VE is an efficiency (usually in percent/100, or unitless) this is an indicator on how well the cylinder fills with new air every time the cylinder draws down. • Larger VE values means more air (mass) enters cylinder compared to smaller VE values. • It is possible to have VE values greater than 100%, this just means the air is more dense than what the theoretical numbers on the denominator indicate. • There are different reference points for VE – for example the use of ambient air pressure/temperature instead of manifold. MegaSquirt uses manifold-referenced values. • But … what drives the changes in VE?
Lets go to the tin can with ball bearings analogy again…. What if part of the can was filled with rocks and gravel? The gravel displaces the amount of ball bearings that get in – and gravel does not burn… The gravel is known as Residual Gas
Mass Air Estimation - VE • VE is largely influenced by residual gas left from the previous combustion event. One would think that the piston would expel all gas during the exhaust…. Think again… • Residual gas is the “leftover” exhaust gas that displaces good fresh air. • However, during the valve overlap period the intake manifold pressure is often less than the exhaust pressure – and this causes a backwards flow of exhaust gas. • Additionally, exhaust backpressure can affect residual gas – and this can be altered by barometric pressure, turbo wastegate, VVT changes, etc.
VE change from wastegate operation due to backpressure change Per Andersson Thesis 934
Mass Air Estimation - VE • Other players in VE: • Fuel vapor pressure – remember that fuel also take up space. • Charge heating in the manifold/cylinder can change the air density – lower mass air flow rates give longer time for the air to “heat up”. • Sonic flow thru throttle plates (isentropic). • “Induction ram effects” • All of this means that the amount of new air that gets into the cylinder is scaled by VE. • VE can be mapped as a function of engine speed and manifold pressure.
Mass Air Estimation - VE • The earlier equation is now modified with the VE term: Volumetric Efficiency, as function of RPM and Pman
Mass Air Estimation - Rate • Note that in all of the above we have been viewing the cylinder in terms of one engine fill cycle. This view is simple to understand and visualize. • On a running engine there are lots of cylinders filling up, and the faster the engine runs the more filling that happens. • Here is a relation that converts mass air (Ma) into terms of mass air flow (MAF):
MegaSquirt Fueling • We now have all of the pieces required for determine the amount of fuel required to match the air filling the cylinder at a ratio we define. • A lot of the variables are constants, like cylinder volume, injector flow rate, and the like. And – everything is either multiplied or divided together, we are dealing with scale factors and ratios. • What MegaSquirt does is define a variable called REQ_FUEL that pulls all of the non-changing pieces together into one number. • All of the previous equations are at work but simply restructured in order to apply simple scale factors.
MegaSquirt Fueling • Definition of REQ_FUEL is the amount of time (milliseconds) the injector needs to be open to deliver the proper amount of fuel specified by the given air-fuel ratio, at 70 DegF, MAP of 100KPa and VE of 100%: