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Discussion Starter #1
any body know of any good sites that might have some good information and pics of the internal combustion engines? any help with this would be great i have tried searches online but nothing has really gave me what i am looking for or at least nothing very good. any help would be greatly appreciated

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very simply explained and great pics/animation.

13,267 Posts
Engine Inefficiency
Kevin Cameron

Unacceptable Losses

Every engine loses part of its power to friction. Pistons and their sealing rings must slide on oil films whose viscosity resists that sliding. Journal bearings likewise shear the oil films that support them. Power is taken from the engine for the processes of emptying and filling the cylinders. Cams bear hard against tappets as they open and close valves. Oil leaving crankshaft bearings is hit and accelerated by the moving parts, creating further losses. Just how big are these losses? In well-designed engines a typical figure is 15 percent of the output power. It’s hard to reduce this figure by much because moving parts have to be supported by oil films. Oil viscosity - necessary to support moving parts - also creates friction as it does so. In unsupercharged engines, power increases are often sought through higher rpm, but if friction loss exceeds 15 percent, its rise with rpm can cancel the gains. Friction arises from the loads on moving parts. Part of this load is the force of combustion gas on the pistons. Another part - often larger than gas loads in high-rpm engines - comes from the inertia of the parts themselves. At peak revs in a sports or race engine, pistons must be stopped from 100 mph, reversed, accelerated to 100 mph in the opposite direction and stopped again. This occurs hundreds of times per second. Peak piston acceleration reaches thousands of Gs. That creates large inertia loads on cylinder walls (because rod angle wedges the piston against the side of the cylinder) and on main and rod bearings. Cams must accelerate valves rapidly to open them as fast as the piston’s motion requires.

Engineering’s response to these loads has been to make moving parts lighter, thereby reducing inertia forces. Pistons used to look like pails, but now they look like ashtrays. Where once thick, heavy piston crowns conducted heat from the hot dome center to the cooler cylinder walls, now oil jets located in the crankcase must cool lighter modern pistons. Piston rings have become steadily narrower, and in race engines only a single gas-pressure ring is used instead of the two found in production engines. Con-rods once had thick, beefy shanks, but now they are slender and much lighter. Titanium, which is only 60 percent as heavy as steel, is used as rod material in many racing engines. Valve stems have become half the diameter they once were, and they are therefore only one-quarter the weight. Because journal-bearing friction rises as the cube of diameter, designers make crank bearings as small as possible. Deep-oil sumps or dry-sump systems with remote oil tanks keep oil away from the crank, and perforated crankcase screens stop flying oil to allow it to drain meekly back to the pump, rather than bounce off the crankcase’s inner surfaces to hit the fast-spinning crank again. In engines with oil scavenge problems five horsepower can easily be lost, and the resulting heating of the oil causes viscosity loss.

Because normal production oil-control piston-rings need heavy wall pressure to work properly, other means of oil control have been sought. Many race engines now employ high-volume air pumps to rapidly extract oil droplets from the crankcase, leaving much less for oil rings to do. Oil rings of lighter wall pressure can then save some power.

Japanese makers of motorcycle engines say that much of the power they have gained in the past 20 years has not come from hot-rod techniques, such as high compression and longer valve timing, but from careful, detailed friction reduction work. In a very real sense, such power gains are “free.” Early Japanese four-cylinder, in-line engines had six main bearings, with the extra one being provided to better support the central cam drive sprocket. One step has been to revert to a more normal setup with five main bearings by moving the cam drive to one end of the crank - where it is on auto engines. Another has been to adopt a V4 engine architecture, pairing rods on only two crankpins and reducing the number of main bearings to three.

Hey, why not build engines the way they used to - with all ball, roller and needle bearings? Wouldn’t that cut friction way down? As snowmobilers know, rolling bearings do make it easier to turn a cold engine, but once warmed up and running in the powerband, back-to-back testing has repeatedly shown there to be little difference between an all-rolling-element engine and the current journal-bearing designs. And there are real advantages on the journal bearing side. First off, the crank can be made in one piece, making it immune to the twist and misalignment problems of built-up roller cranks. It’s stronger, too. Secondly, nothing is smaller or lighter than the thin split shells of a journal bearing, but balls and rollers require thick, heavy and bulky steel outer races. Lastly, journal bearings win on grounds of strength and fatigue resistance. Instead of the concentrated line or point contacts of rolling elements, journals spread the load over a large area. They also supply superior shaft damping.

Anyone who has tried to turn the crankshaft of a four-stroke engine is impressed with the large torque required. It seems impossible that so much resistance can be associated with a high-revving engine, and in fact it is not. When you turn the crank by hand, speed is too low to form full oil films, so you feel the heavy resistance of piston rings sliding on bare metal walls and the lumpy torque required to drive the valves. Once the engine starts and oil is circulated by the pump, this friction drops rapidly, and the engine does in fact spin easily.

Of course two-stroke engines must use rolling bearings because, while flowing their mixture through the crankcase, they cannot employ the recirculating pressure oil system that journal bearings require. Rolling bearings thrive on very little oil. When you think of the lubrication of a ball or roller bearing, think of a car or truck tire running at high speed on a wet road. Water sprays from the tire in all directions as it mashes aside the water layer ahead of it. This kind of high-speed fluid pumping eats power and generates heat. It is to avoid loss of this kind that a fine oil mist provides the best lubrication for rolling bearings, as they are in two-stroke engines.

Two-stroke friction loss is generally less than that of four-strokes for two reasons. First, the engine has fewer moving parts since it lacks camshafts and valves. Second, the use of the crankcase as a scavenge pump equalizes the pressures above and below the pistons, reducing part-throttle pumping loss.

If you graph out friction loss versus rpm in four-stroke engines, the curve has a U-shape, sometimes referred to in the biz as a “bucket.” In the “bottom of the bucket,” at low speeds just above idle, friction is at a minimum. As the engine accelerates, inertia forces rise as the square of rpm, so friction rises steeply. If the engine is slowed to idle speed, friction begins to rise because, at such low speed, oil films have time to squeeze out from under heavily loaded parts like cam lobes and tappets. Instead of the .001-.002 friction coefficient of a full oil film, local rubbing now takes place on the solid additive films deposited on parts from the oil. This solid friction is of the order of .01-.02, or 5 to 10 times greater. For this reason, friction rises at very low rpm. In fact, engines have, through human error, been built so they would not idle at an acceptably low speed - because the low-speed friction rise was too large, which leads to stalling. A heavier dose of anti-wear additive in the oil (special to this model) fixed this.

There are three regimes of mechanical friction. The first and most desirable form is full film, or “hydrodynamic” lubrication, in which the moving part moves fast enough to generate a full oil film that carries the load. In this case, friction is determined by the viscosity of the oil, the load and the speed of movement.

The opposite extreme is boundary lubrication, in which an oil film is no longer present and the parts are separated only by whatever oxide and oil additive-derived films cling to parts. Such films take many forms but serve to protect parts at times like cold-start, after oil has mostly drained off of moving parts during shutdown. An intermediate form is mixed lubrication, in which there is partial oil film support, assisted by surface chemical films.

Most of the friction in a running engine is of the first kind - viscous friction in full oil films. Parts such as piston rings and cam lobes move fast enough to generate full oil films most of the time. The exception is that near TDC, between the compression and power strokes, there is large pressure on piston rings from combustion gas, but piston velocity is low. This is the reason for accelerated wear of the ring track right at the top of ring travel. Oil additives such as the widely used anti-wear ZDDP provide valuable protection in such cases.

Engine and parts flexure is another source of friction, causing bearing misalignment and unintended heavy loading. When crankcases have been redesigned for greater rigidity, actual power increases have often resulted.

Much of the wear that occurs in engines happens in the first few seconds after start-up, as oil has previously drained from moving parts, and it takes seconds for pumped oil to reach the most distant points. Especially harmful is the immediate revving and loading of engines in those first few seconds before oil pressure arrives. This is why the oil systems of expensive racing four-stroke engines are sometimes pressurized externally before they are started.

A common misconception is that merely by moving back and forth, parts like valves and pistons somehow use up lots of power. This is not true. As a piston is pulled to a stop by the rod at the top of its travel, its kinetic energy is thereby transferred to the crankshaft. As the rod then pulls it suddenly downward, the power to accelerate it is taken back from the crank. The only losses involved in this back-and-forth exchange of energy between piston and crank are in the sliding contact of piston and cylinder and in the associated rod and crank bearings.

The same is true in the valve train. The cam lobe lifts a valve, compressing its valve spring. This energy is not lost but is stored in the spring. As the cam lobe turns further and the valve is permitted to close, the energy stored in the spring pushes against the closing flank of the lobe, exerting a forward torque on it that returns the stored energy to the camshaft. The only energy lost in this process is that consumed by friction, mainly between cam and tappet.

Many studies have been performed in which engines are driven in various stages of assembly with an electric motor to measure the required power. Such studies reveal that piston and, particularly, ring friction make up the largest part of the overall 15-percent friction loss, with crank and rod bearings next, while valve gear and accessory drives are further down the list.

Oil itself has become a tool in friction reduction, mainly through reductions in viscosity. The practical limit to how far oil viscosity can be reduced to save friction is set by the roughness of bearing surfaces. Seen under high magnification, even the most polished surface is covered with asperities - tiny irregularities that project upward. When the oil film separating parts becomes thin enough, the tips of these asperities on mating parts’ surfaces begin to touch each other, weld together, and then pluck material that creates wear particles. In heavily loaded bearings, minimum oil film thickness can be as little as 40-60 millionths of an inch.

Stock car racers began to have crankpins “superfinished” by a lapping process once used to shorten the break-in of combat aircraft engines. This enabled them to safely use lighter oils. Superfinished crank journals are now widely used in production engines. Cylinder walls - once honed to a rough 220 finish to promote fast break-in of piston rings - are now “plateau finished” to a much finer 600 finish, while piston rings are factory prelapped to ensure quick sealing in new engines. A plateau finish begins with a rougher hone to create a crosshatch pattern of grooves. Finer hone stones are used next, creating a very smooth surface interrupted by the remains of the grooves. Any tendency to scuff is quickly stopped as the debris reaches one of the grooves and falls into it.

These smoother finishes on late-model engines allow the maker to specify use of lower-viscosity oils, which in turn cut friction by a few percent. Remember that a claim such as “Cuts friction by up to 3 percent” is not the same as “Increases power by up to 3 percent.” Friction is only taking about 15 percent of an engine’s power. Three percent of that 15 percent is a power increase of a bit less than half a percent.
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