intravenous
2008-11-01, 06:05
The mechanical efficiency of an engine is the ratio of the useful work at the crankshaft to work done on the piston, and varies between 80 and 90%. Useful work can be measured with a dyno, ie horsepower, kilowatts, whatevers, while work on the piston can be obtained from an indicator diagram and is called indicated hp. It's also worth knowing that (i.m.e.p) = indicated mean effective pressure, and (b.m.e.p) = brake mean effective pressure.
In a 4-stroke engine, effective pressure, ie pressure producing rotation, is only applied on one stroke in four, so one stroke every two revolutions of the crank. This pressure varies greatly, from a maximum of 900lb/sq at the beginning of the power stroke to something like 100lb/sq by the time that the exhaust valves open.
The i.m.e.p is the average pressure in lb/sq in the cylinder during the whole of the working cycle. The b.m.e.p is the average pressure per sq in the cylinder which is converted into useful work. The difference between these two figures is a measure o the mechanical losses reckoned as pounds per pressure per sq in.
b.m.e.p/i.m.e.p x 100 = the mechanical efficiency of the engine in question.
Brake horsepower can be easily calculated, and from it the b.m.e.p.
One hp = 33 000ft lb per minute.
P = b.m.e.p in lb/sq in
A = area of piston in square in
S = stroke in inches
n = crankshaft revolutions per minute
k = power strokes per revolution
Then bhp = (ASnkP/33,000) x 12
And P = (bhp x 33, 000 x 12)/ASnk
The total mechanical losses in an engine can be measured by motoring it around at different speeds and observing thr hp absorbed in doing so. Add this to the bhp figure and the answer is the ihp, which allows us to work out the i.m.e.p in the same way I just showed you how to work out for b.m.e.p.
I've gone through this shit because it is useful for comparing different engines. A hp figure doesn't really tell us much about how good or bad an engine is in comparison to another, but if we have the power curves, and we know the bore of stroke of the engines that we are comparing, a bmp curve can be generated, giving a real basis for comparison. It also gives a good understanding of all the factors upon which useful power production depends.
The mechanical losses to be overcome in an engine are caused by:
-piston friction.
-bearing friction and accessory drives.
-cylinder pumping.
-oil drag in the crankcase.
-lack of balance (as I've posted a wee bit about before).
The greatest of these losses to overcome is piston friction (that's why I put him at the top of the list :D). Piston friction is the result of shearing the oil film, and that reduction of the area of the film, that is, the contacting area of the skirt, will reduce the friction.
Since piston thrust is obviously taken in the place at right angles to the crankshaft, a bearing surface on the sides of the piston is not necessary to its proper functioning. From this, coupled with the desireability of reducing the contacting area came the slipper-type piston.
Due to the angularity of the connecting rod, a thrust is set up between piston and cylinder under explosion pressures and, at other points in the cycle, under inertia forces. At high speeds, inertia forces become greater than your explosion forces, and as, at a given speed, the inertia is proportional to the weight, it will be clear that the reciprocating parts (piston, gudgeon pin, little end of conrod) should be as light as you can get away with, while still being strong enough to do the job. This also reduces bearing loads and therefore bearing pressure. The extent to which the skirt of the piston can be removed without the bearing pressures becoming so high as to squeeze out the oil film and cause a seizure has not be fully investigated, but you can take a lot of material off most pistons.
The type and condition of lubricant also have an effect on piston friction, but I'm too fuckin' high to want to write about that boring shit. Tribology can suck my balls.
Ah fuck, I better mention it. A piston works under very exacting conditions. It cannot be lubricated in the same way that a big-end bearing can, because too much oil would pass to the combustion chamber, and on each power stroke the film of oil on the cylinder walls is exposed to very high gas temperatures, which partially burn and destroy its lubricating properties. Generally, the thicker the oil the better it withstands heat, but it creates more friction.
Piston clearance also affects piston friction. Aluminium-alloy pistons require around 0.0004in. clearance per inch diameter measured at atmospheric temperatures at the top of the skirt. The smallest clearance consistent with no loss of power is desireable, to avoid tilting and thus maintain the piston rings in proper contact with the cylinder. In the early days of alloy pistons, too little clearance resulted in immediate seizure, but with the hard alloys that we now have, you can lose power because off too close a fit but not have a seizure or any real signs of the piston picking up along the bore.
Next after the losses from piston friction are the pumping losses. These are generally incurred on the induction stroke. Technically they aren't a mechanical loss at all, but they are included because they are a result of the function of valve and port dimensions. The lower the gas velocity through the inlet valve the smaller the loss, which is one of the reasons for the use of multiple valves in an engine, instead of two big ones. There is only a slight pumping loss on the exhaust stroke because the gases are released under pressure.
The most important point under the heading of 'bearing friction and accessory drives' is valve operation. The valves have to be opened many times a second against spring pressure, througha distance equal to the theoretical valve lift minus the working clearance. Some of the work done in opening is regained on closing, when spring pressure tends to push the cam around in direction of it's rotation, but this proportion is not great and it is usual to consider that all the work is lost. The actual loss though can be calculated relatively easily.
Lift of valve = 3/8in.
Average of spring pressure (spring pressure at half-lift) = 180lb
Speed = 6000rpm
Each valve will lift once every two revolutions of the crank, therefore, one movement per revolution.
Work done per minute = (6000 x 180 x 0.375)/12
Therefore, hp = (6000 x 180 x 0.375)/(12 x 33, 000)
= 1 :)
There is also extra work done in lifting the exhaust valve against gas pressure, but this is fairly small because although the load is high, it falls immediately the valve lifts. Friction between cam and follower surfaces is another source of loss, the magnitude of which depends on spring pressure, cam contour and quality of lubrication. The reduction of weight in the valve operating gear is of utmost importance, as spring pressure may be reduced in nearly direct proportions. This is where an overhead cam engine shines over a pushrod type really, because the weight of the reciprocating valve gear is reduced by the weight of the pushrods.
That's all for now. IV is stoned, sleepy and hungry. He needs some more uppers.
In a 4-stroke engine, effective pressure, ie pressure producing rotation, is only applied on one stroke in four, so one stroke every two revolutions of the crank. This pressure varies greatly, from a maximum of 900lb/sq at the beginning of the power stroke to something like 100lb/sq by the time that the exhaust valves open.
The i.m.e.p is the average pressure in lb/sq in the cylinder during the whole of the working cycle. The b.m.e.p is the average pressure per sq in the cylinder which is converted into useful work. The difference between these two figures is a measure o the mechanical losses reckoned as pounds per pressure per sq in.
b.m.e.p/i.m.e.p x 100 = the mechanical efficiency of the engine in question.
Brake horsepower can be easily calculated, and from it the b.m.e.p.
One hp = 33 000ft lb per minute.
P = b.m.e.p in lb/sq in
A = area of piston in square in
S = stroke in inches
n = crankshaft revolutions per minute
k = power strokes per revolution
Then bhp = (ASnkP/33,000) x 12
And P = (bhp x 33, 000 x 12)/ASnk
The total mechanical losses in an engine can be measured by motoring it around at different speeds and observing thr hp absorbed in doing so. Add this to the bhp figure and the answer is the ihp, which allows us to work out the i.m.e.p in the same way I just showed you how to work out for b.m.e.p.
I've gone through this shit because it is useful for comparing different engines. A hp figure doesn't really tell us much about how good or bad an engine is in comparison to another, but if we have the power curves, and we know the bore of stroke of the engines that we are comparing, a bmp curve can be generated, giving a real basis for comparison. It also gives a good understanding of all the factors upon which useful power production depends.
The mechanical losses to be overcome in an engine are caused by:
-piston friction.
-bearing friction and accessory drives.
-cylinder pumping.
-oil drag in the crankcase.
-lack of balance (as I've posted a wee bit about before).
The greatest of these losses to overcome is piston friction (that's why I put him at the top of the list :D). Piston friction is the result of shearing the oil film, and that reduction of the area of the film, that is, the contacting area of the skirt, will reduce the friction.
Since piston thrust is obviously taken in the place at right angles to the crankshaft, a bearing surface on the sides of the piston is not necessary to its proper functioning. From this, coupled with the desireability of reducing the contacting area came the slipper-type piston.
Due to the angularity of the connecting rod, a thrust is set up between piston and cylinder under explosion pressures and, at other points in the cycle, under inertia forces. At high speeds, inertia forces become greater than your explosion forces, and as, at a given speed, the inertia is proportional to the weight, it will be clear that the reciprocating parts (piston, gudgeon pin, little end of conrod) should be as light as you can get away with, while still being strong enough to do the job. This also reduces bearing loads and therefore bearing pressure. The extent to which the skirt of the piston can be removed without the bearing pressures becoming so high as to squeeze out the oil film and cause a seizure has not be fully investigated, but you can take a lot of material off most pistons.
The type and condition of lubricant also have an effect on piston friction, but I'm too fuckin' high to want to write about that boring shit. Tribology can suck my balls.
Ah fuck, I better mention it. A piston works under very exacting conditions. It cannot be lubricated in the same way that a big-end bearing can, because too much oil would pass to the combustion chamber, and on each power stroke the film of oil on the cylinder walls is exposed to very high gas temperatures, which partially burn and destroy its lubricating properties. Generally, the thicker the oil the better it withstands heat, but it creates more friction.
Piston clearance also affects piston friction. Aluminium-alloy pistons require around 0.0004in. clearance per inch diameter measured at atmospheric temperatures at the top of the skirt. The smallest clearance consistent with no loss of power is desireable, to avoid tilting and thus maintain the piston rings in proper contact with the cylinder. In the early days of alloy pistons, too little clearance resulted in immediate seizure, but with the hard alloys that we now have, you can lose power because off too close a fit but not have a seizure or any real signs of the piston picking up along the bore.
Next after the losses from piston friction are the pumping losses. These are generally incurred on the induction stroke. Technically they aren't a mechanical loss at all, but they are included because they are a result of the function of valve and port dimensions. The lower the gas velocity through the inlet valve the smaller the loss, which is one of the reasons for the use of multiple valves in an engine, instead of two big ones. There is only a slight pumping loss on the exhaust stroke because the gases are released under pressure.
The most important point under the heading of 'bearing friction and accessory drives' is valve operation. The valves have to be opened many times a second against spring pressure, througha distance equal to the theoretical valve lift minus the working clearance. Some of the work done in opening is regained on closing, when spring pressure tends to push the cam around in direction of it's rotation, but this proportion is not great and it is usual to consider that all the work is lost. The actual loss though can be calculated relatively easily.
Lift of valve = 3/8in.
Average of spring pressure (spring pressure at half-lift) = 180lb
Speed = 6000rpm
Each valve will lift once every two revolutions of the crank, therefore, one movement per revolution.
Work done per minute = (6000 x 180 x 0.375)/12
Therefore, hp = (6000 x 180 x 0.375)/(12 x 33, 000)
= 1 :)
There is also extra work done in lifting the exhaust valve against gas pressure, but this is fairly small because although the load is high, it falls immediately the valve lifts. Friction between cam and follower surfaces is another source of loss, the magnitude of which depends on spring pressure, cam contour and quality of lubrication. The reduction of weight in the valve operating gear is of utmost importance, as spring pressure may be reduced in nearly direct proportions. This is where an overhead cam engine shines over a pushrod type really, because the weight of the reciprocating valve gear is reduced by the weight of the pushrods.
That's all for now. IV is stoned, sleepy and hungry. He needs some more uppers.