Formulations important to the advanced mechanic.

Volume of physical area.


Cylinder volume is bore times stroke, bore is the diameter and stroke is the length the piston travels. cylinder volume is written in cu.in (cubic inch) or cc's (cubic centimeters), 61 cu.in = 1000 cc.



Oil pan volume is written in quarts, gallons or cc's. 1 quart is 1000 cc.



A small single cylinder engine in a lawn mower usually holds around 750cc (3/4 quart) to 1000cc (1 quart) of air cooled engine oil. A riding mower engine one or two cylinder air cooled engine usually holds around 1500cc to 2000cc or two quarts of engine oil. A water cooled larger engine found in most automobiles hold around 4 to 5 quarts of engine oil and a large truck engine holds around 5 gallons of engine oil. 



Volume of liquid fuel is written in gallons, a gas fuel is written in gallons also. The flow of fuel is written as Gpm  (Gallons per minute) or Lpm (Liters per minute). The flow of a gas fuel or air is cu.ft./m (cubic feet per minute).



An automobile that gets 30 miles per gallon of fuel and going 60 mile per hour will use 2 Gph (Gallons per hour) or .033 Gpm (Gallons per minute) so calculations regarding heat generated and BTU potential for efficiency purposes can be based from volume in cu.in. and horsepower developed by the engine. Efficiency of an internal combustion engine using fuel injection is far better than the efficiency of an engine using a carburetor.



When carburetors were used for an internal combustion engine you could expect around 13 to 16 miles per gallon from a 350 cu.in displacement engine in a pickup or car weighing around 4000 lbs. 16 Mpg if lighter and 13 Mpg if heavier. At that time the fuel to air ratios for a great tuned engine was around 17 to 1 (17:1) but now with fuel injection the ratio is higher and around 19 or 20 to one, that is 19 parts of air to one part of fuel and the part size does not matter because it is the ratio which is a constant.



Mechanical ratios are specific and there are several to know about when talking about engine fuel ratios, engine compression ratios, engine to gear output ratios, differential gear ratios, and engine pulley ratios.


Fuel ratios are simply how much air is mixed with one part of fuel, NOTE that a part of fuel could be one droplet, one pint, one quart, or one gallon and so on. The volume of air it is mixed with is calculated as the same volume of the fuel so the parts are identical in calculations. If we have a fuel to air mixture of 17 to one (17:1) and the engine is running good we can bump the fuel ratio to 19:1 which is much leaner and if the engine still runs good then we can run 19:1 and use less fuel per mile traveled. The leaner the fuel to air mixture the hotter the engine will run at, so at one point the engine will overheat, run bad and pop back through the intake manifold if the fuel to air ratio is to high.

Compression ratios are simply calculated in the same way, the hollow cylinder has a volume of area that air and fuel can combine within. A compression ratio of 9 to one (9:1) simply squeezes the air fuel volume of one cylinder into an area 9 times smaller, and the area within the cylinder then builds up pressure inside to an average of 200 lbs. per square inch. Most old engines would run compression ratios of 8.5:1 to 9.8:1 (8.5 to one - 9.8 to one) on regular fuel.



When working with fuel to burn the first lesson needs to be spontaneous combustion, at what temperature will the fuel start burning without an external source of electrical spark or an additional ignition source. A gasoline engine is designed to require an electrical spark to start the fuel burning. Let's just grab an even number and say the fuel we are using is regular or around 80 octane and begins to self ignite at 350 degrees F. Each pound of compression raises the air temperature around 2 degrees F. So with those numbers we can see that if our cylinder develops 200 lb. compression the fuel and air reach a temperature that will self ignite the fuel. When this happens the cylinder then fires off before it gets to the point it is supposed to be when the electrical spark comes from the coil. This creates loud noises like a pinging, a knocking, and overheats the engine. This can damage the engine to permanent failure. So the compression ratio determines the fuel required to safely run an internal combustion engine.


For example, a regular engine having a compression ratio of 8:1 and up to 9.8:1 and having a total compression pressure of 175 psi. can run regular fuel lower octane, 80 to 85. From 9.8:1 to 10.5:1 regular high octane pump gas can be used 85 to 95 and a high compression engine 10.8:1 to 16:1 needs as high as can be, racing fuel to 130 octane. Octane is a rating that enables the fuel to reach a higher temperature before self ignition, the gas manufacturers use additives to slow down the burning process of a fuel. At 16:1 or higher straight alcohol will run good and a diesel engine has a compression ratio of around 22:1.



When you want to develop more power at a lower RPM you raise the compression ratio in the cylinder, this can be accomplished by cutting the surface of the engine head that contacts the cylinder top surface where the gasket is placed (head gasket). You can also raise the compression ratio by installing a piston with a higher dome on it. Nonetheless raising the compression ratio in an internal combustion engine makes more power at a lower RPM and this is called torque. Important to consider is the availability of good fuel, it is not advised to run a regular fuel in a high compression engine so more power is going to cost more to operate. It's just how bad do you want more engine torque.



So your engine is simply an air pump, the more air you can pump through it the more power you will get if the fuel ratio is correct. Fuel to air ratio of 17:1, compression ratio of 10:1 and you could take it to a dirt track and compete.



Gear ratios are calculated normally by counting the teeth but another way is to measure the outside dimension of the drive gear (driven by an engine or primary mover) and a driven gear (driven by contact from the drive gear). When the drive gear is one inch diameter and the driven gear is four inches outside diameter the ratio is said to be a 4:1 meaning the drive gear rotates four complete revolutions and the driven gear rotates one time around. So the driven gear then is connected to an output driving a shaft or wheel called the final drive.



For example a camshaft and a crankshaft in an internal combustion engine have a gear ratio of 2:1. The engine crankshaft rotates one time and the camshaft goes 1/2 a rotation. The crankshaft rotates twice around and the camshaft rotates once around.



For pulley systems the outside diameter is what is used to calculate ratios. It is a good idea to always rate ratios from the primary mover to the final drive. To understand more regarding a primary mover click on (this link) .   

Any primary mover can be considered an air pump, the more air you can pump through the cylinders the better it will perform until you reach the point that the metal parts begin breaking. Enhancing an engines performance simply requires more air and fuel to be burned efficiently. One way is to make it rotate faster, the need then becomes structural metal because of added force by rotating more RPM than was originally designed into it.



The fundamental requirements to make any engine perform better than stock, (stock is the way it came when purchased originally) are simply removing air restrictions from the intake air system to the engine head and from the exhaust system from the engine head out to the tailpipe. Make sure the fuel mixture is burning cool enough for the spark plugs to look a very light tan color, not black and not white, the old guys call it chocolate brown (around 17:1) and will allow the engine to feel more powerful than the totally white plugs. The spark plug end that is in the cylinder has the white porcelain and end tip, this is the part that want to be slightly discolored into the tan range. Adjust your fuel to air mixture until you see the slight discoloration on the spark end of the spark plugs.

Going deeper you can change the cams, port the intake and exhaust tubes and this makes more air flow through the engine. Putting stiffer valve springs in makes the cylinder seal air in better and helps on the low end giving a feeling of more power down low when accelerating, also the valves will come back to the seats quicker making it possible to turn higher engine speeds. So high performance is all about getting more air and fuel to burn efficiently.

When calculating air flow in and engine take the displacement of the engine cylinders and divide it by 2. Multiply that sum by the desired RPM of the engine at top performance. For example a 350 cu.in. V-8 needs a minimum of 175 cu.in. of air to fill 4 cylinders each revolution of the engine. Now you need to multiply that by 6800 for a top performing V8-350CID (cubic inch displacement) 1,190,00 cubic inches of air every minute to simply fill the cylinders. In one cubic foot there are 12 x 12 = 144 x 12 = 1728 cubic inches so 1,190,000 divided by 1728 = 689 cubic feet per minute @6800 RPM. That is how you size your intake and exhaust system to flow through that amount and I always add in 18 to 20% over when sizing air flow for high performance prime movers. 

An engine cam shaft rotates at a ratio of 2:1 ( 2 revolutions of the crankshaft = 1 revolution of the cam). The cam opens the engine valves and the valve springs bring them back to their seats. A seat is the area in the cylinder head that is cut so the valve seals air and fuel inside the combustion chamber. The combustion chamber is sealed air tight on the compression stroke and allows the burning fuel to exert pressure on the dome of the piston.



Ratios are found in the movement of the rocker arms, the compression of an engine, the fuel to air mixing, the torque converter, the engine pulleys, the transmission, the differential and so on, ratios are critical to the formulation of a good running engine.



We are bombarded with ratios in life any time there is a transition between input and output, the differences are calculated in ratios. When I put in $10 dollars in fuel and get ten miles distance that ratio is a 1:1, one gallon fuel to one mile so ratios can be applied to any type of work related issue. The important fact here is understanding the order of a ratio, from the primary source of the input power to the driven member is what your ratio needs to begin with.

Formulas used daily in a repair shop or engineering department:


1)  One millimeter = .03937 inches.


2)  One horsepower will produce 746 Watts.


3)  One horsepower will produce one gallon of fluid flow @1500psi.


4)  Two horsepower will produce one gallon of fluid flow @3000psi.


5)  746 Watts electrical energy will power a 1 horsepower motor.


6)  Watts is volts (E) times amps (I).


7)  Amps are volts divided by ohms (R).


8)  Each cell in a lead acid battery should produce 2.2 volts.


9)  Torque is calculated in foot pounds.


10) For each pound of compression the internal heat rises 2 degrees.



For Example: As an advanced mechanical engineer these formulations are the basic internal combustion prime mover formulations for optimizing performance. First you decide how much horsepower you need to do your work, it may be an automobile or a motorcycle engine to make it faster or better or even more economical on fuel. If you want your vehicle to go faster quicker weight needs to be removed, more air and fuel need to be able to go through your engine, and things need to be more securely fastened together. An example of making a simple change in fastening things together is an engine mounted to a frame. The engine will produce it's horsepower when the engine is mounted to the frame with conventional engine mounts containing shock absorbing rubber. By mounting the engine solid to the frame allows the engine to translate more horsepower to the driven member by reducing the ability of vibrational changes. The more solid mounts the more transfer of horsepower. The engine is not making more horsepower but moreover the horsepower made is transferring better than when vibration is allowed by rubber mounts.



So we have an engine that works good and winds out normally and goes from 0 to 100 miles per hour in just 10 seconds. The vehicle weight is around 2000 pounds and we need to figure out just what we interpret is running better. Is it more top speed we want, is it more torque to pull more weight quicker, do we want a race engine to turn high RPM for a long time or a drag racing engine turning high RPM for just a few seconds? Those are questions that a person needs to figure out to then go and enhance the performance of the vehicle. Just stating I want it to run better without taking the engine apart means clearly you will be limited to working on intake and exhaust system, fuel system delivery pressure, wheels, ignition spark and electrical timing and the quality of fuel always helps.