Engine Cylinder Design and Function - Part 9

Weird and Wonderful
Unique cylinders

Honda NR500

Over the years there have been cylinders that have stood out because of the way in which they differ from the ‘normal’ hole in a block. Its worth mentioning them here as they show that cylinders can become of a component than just a feature.

First off, in the 1979 Honda went all out nuts and produced the NR500. This was a V4 engine with oval piston, below shows the pistons and rods.

Honda’s reasoning for detaching from the norm was due to the rules of the MotoGP at the time. The 2 strokes of the age were kicking arse and a 4 stroke didn’t have much chance. This pissed Honda off as they had made good progress with 4 stroke designs for their road fleet. To this end they wanted to increase the number of cylinders to create more power, unfortunately the rules at the time stated that engines can only contain 4 combustions chambers.

So back at Honda HQ they decided to make a V8 engine with only 4 cylinder, or be it an engine with 32 valves, giving the engine the flow capacity of a V8 with only 4 cylinders. This venture worked but there were some issues, like the strange piston ring arrangement and the high reciprocating masses involved by have 4 massive pistons.

Honda suffered from a lot of setbacks until they final sorted out all of the bugs and the bike ran pretty well. The V4 500cc engine developed 130 bhp at over 20,000 rpm, which is quite scary. Unfortunately the new bike didn’t win any silverware and they turned there backs on the engine and started developing the NS500 2 stroke which won the 1983 500cc World Championship.

One thing to consider in the realm of cylinders is how they cut the cylinders and more importantly how they achieved a honed surface. The point being is that not all cylinders are, well cylindrical.

RCV engine

The RCV or rotating cylinder valve engine uses the cylinder as a valve. The cylinder is ported like a 2 stroke engine but instead of relying on the piston alone the entire cylinder rotates to cover and uncover ports with the barrel.

 

This in effect is a 2/4 stroke hybrid. In reality it’s a 4 stroke with a valve system similar to a 2 stroke. The entire cylinder is free to rotate with another cylinder. The outer diameter of the cylinder is coupled to a gear that runs off a mitre gear from the crankshaft to keep the cylinder in time. As the cylinder rotates it uncovers ports then allow for induction and exhaust outgassing at the correct moment.

One of the challenges of the design as sealing the cylinder whilst maintaining free rotation. This engine is still under development in the Dorset in the UK, and some engines have been shipped to customers worldwide. The main market at present for these engines at present (2015) is as a UAV powerplant. 

The cylinder itself has become a moving component that controls port timing.

Engine Cylinder Design and Function - Part 8

Stroke
The route of all power


So we’ve looked at many of the different aspects of the cylinder but one thing that we’ve overlooked is the stroke. I’ve previously mentioned the importance of the bore and the stroke. The ‘bore’ is the internal diameter of the cylinder, however the stroke is NOT the length of the cylinder. The stroke is defined by the ‘throw’ of the crankshaft, and is the distance the piston travels inside the cylinder.

Below shows the bore (1) and the stroke (2). Notice how the cylinder is absent from this diagram. Cylinders are always larger than the stroke to guide the piston as it travels. The piston travels to the top of the cylinder, but does not travel to the bottom of the cylinder.

The image below illustrates how the cylinder guides the piston due to the fact that it is longer than the stroke.

  

In some engines, especially single cylinder engines the cylinder protrudes further into the engine casing than the body of the barrel, this is to retain a suitably long cylinder but reduce the overall height of the engine.

Stroke as stated before is determined by the throw of the crankshaft and we’ll go more into this subject in the crankshaft section, its just important to point out at the length of the cylinder isn’t the stroke length.





Porting the cylinder
Cylinder valves 


2 stroke engines differ from there counterparts (4 strokes) due to 2 differences, the cycle of the system and the use of the cylinder to control the fluids of the system. This might sound like technical bumf, and…….. well it is. What is meant by this is that the 2 stroke complete 2 strokes (once up and down) and the whole cycle starts over again. The fluids of an engine mean both fuel and air, either as a mixture or not.

2 stroke engine have port cut into the sides of the cylinder that allow air/fuel to pass into the cylinder and back out again at the appropriate time. For now we’re looking at cylinders so we’ll skip all the port timing nonsense until later. What is important is that the cylinder plays a major role in the 2 stroke design unlike the 4 stroke were it simply functions as a hole.

Ports in a 2 stroke petrol cylinder.

There are 2 major types of cylinder porting used in combustion engines. These are single flow diesel 2 strokes and multiple porting used on most petrol 2 strokes. This porting arrangement isn’t so much to do with the difference between the fuels themselves but the speed at which the engine operates.

  

2 stroke Diesel

 The massive Wärtsilä-Sulzer RTA96-C its 89 feet long and 44 feet wide, 

1,820 liters per cylinder and produces around 107,000 horsepower.

2 stroke diesel engines are usually reserved for heavy machinery and ship engines. The largest engine in the world is a 2 stroke diesel pictured above. These engines also have poppet valves like 4 stroke engine and have ports at the bottom of the cylinder just above the piston at BDC (bottom dead centre).

The function of these port is to fill the cylinder with fresh air when the piston reaches BDC, this is controlled by the piston which uncovers the ports at it bottoms out. The exhaust gases escape though a poppet valve which resides in the cylinder head. These engine are very efficient, hence there main application is to power large ship that run for weeks. 

Above is a liner from a 2 stroke diesel that has been cut away, the piston is also present to demonstrate the uncovering of the ports. This system work very well as the diesel fuel is more of an oil than a solvent and lubricates the cylinder unlike 2 stroke petrol that require an oil to be added to the fuel. Unlike petrol engines only air flows through these ports, as the diesel is directly injected into the cylinder. More on diesels later.

Ported Petrol

Nearly all 2 stroke petrol engines use both exhaust ports and transfer ports to move fluids in and out of the cylinder. Some of these ports have quite complicated paths that direct the flow in and out more efficiently. The number of ports change from engine to engine and as research into 2 stroke engines progresses these numbers seem to keep on increasing.

It is becoming more common for engines to have Aluminium barrels with Nikasil coated cylinders. This combination is to give good thermal transfer and a hard wearing bearing surface.

Small radio controlled aircraft that usually run on petrol or methanol have cylinder liners that are ported. They are usually made from brass with a chrome plated cylinder wall for extra flight hours. This chrome surface is cheaper than Nikasil and will easily outlive the engine.

 

Engine Cylinder Design and Function - Part 7

Ring Ridges
The wear that isn’t there
  
The piston doesn’t travel the entire length of the cylinder, be it sleeved or in-block. The two main causes of cylinder wear are piston slap, which we’ve covered and ring wear. The piston rings eventually wear away at the cylinder and wear themselves. However when rings fail people usually just replace the rings and sometimes the piston, but rarely the entire cylinder, especially the multiple cylinder engines. This means that the cylinder will wear through 2 or even 3 sets of rings before that receive any attention.

Cylinder with heavy ridging, carbon has not been cleaned from this surface highlightening it.

This wear can be corrected, as in some cases has to be. The ring ridge starts where the pistons stops on the upstroke. Most engines are used in transportation and rarely rev high enough for this to become a problem. However, unbeknownst to most people connecting rods stretch under heavy load at high rpm. If the ridge is deep enough and an engine is revved hard under load the rings will pass over this ridge and this sudden impact can break piston rings causing major damage to the cylinder and piston.

 FAE of 3 different styles of connecting rods. This analysis is focused on rod stretch.

To remove the ridges from a cylinder the inner walls need to be machined. This is done with a cylinder boring machine obviously. These machines are self-centering and bore the cylinder to a larger diameter. Usually a oversized piston is fitted to fit take up the slack, most manufacturers sell oversized pistons and rings in 0.50mm increments.

Boring is also the method to remove the oval out of worn cylinders. With sleeved cylinders the sleeve is usually replaced.



 
Honing a bore
Smoothing out the cut

After the cylinder has been bored out to a larger diameter the surface is too rough for piston rings to be fitted. The inner cylinder wall needs to be smooth to ensure good sealing and smooth running. So you’d think the cylinder would be polished to a shine, this would actually be bad for the engine causing excessive heating and probably a seize.

Honing marks in a cast iron sleeve.

The surface also has to be able to retain an oil film to reduce wear. This is where honing comes in. Honing is basically precision grinding, but removes very little material. The bore is machined close but not to the final dimension. This excess of material is left for the honing process.

Honing uses stones of various grip that are fitted to a spring loaded tool that rotates inside the cylinder bore. These stones lightly grind at the inner surface. The honing tool is moved up and down the bore similar to the movement of a piston. As this is happening the honing stones are rotated constantly. The rotating and up and down stroking ensures the stones grind a perfect bore with parallel walls. 

Honing head with stones attached.  

The stroke rate of the honing head is carefully controlled to grind the cylinder in a particular pattern, this is know as cross hatching. Cross hatching is usually at 45˚ along the cylinder wall as pictured above. This angle helps retain oil on the cylinder wall. Too step and angle and the oil will run off the cylinder wall. Too level and the rings will flutter along the bore. Some engines run 60˚ hones with good results, although the angle is harder to control.

 Flex-hone

Other tools available are known as flex-hones, however they should be renamed as they don’t really hone to cylinder wall in the same fashion as a proper cylinder hone. These flex hones are ceramic balls attached to wire brush attachment. They and then fitted to a hand drill and spun in the bore in a similar fashion to a honing head.

Flex hones are only good for removing carbon build up and some cooked on oil residue which is called glazing. Flex hones shouldn’t be used as a substitute for professional honing.
 

Engine Cylinder Design and Function - Part 6

Cylinder Sleeves
Replaceable cylinders 




Some engines are fitted with sleeves also called cylinder liners. These are usually machined from cast iron and are replaceable. Cast iron is a very good bearing surface, this is why a lot of piston rings are made from cast iron. Once the cylinder has become worn the sleeve is removed and a new one is fitted. Piston rings and pistons are usually replaced at the same time to increase the life of the engine. 

Set of 4 liners, machined from cast iron.

The sleeves are machined to be slightly larger than the bored hole for it to fit into. The engine block is then heated so the hole expands. The sleeve is then pressed into the hole. 

 A 2 stroke liner being pressed into the cylinder.

Sleeves are usually fitted with ‘o’ rings to seal them in the block. Sleeves were usually reserved for 4 stroke engines but some 2 stroke engines also have sleeves. These are a bit more complicated as the ports have to line up with the sleeve.

2 stroke sleeve, with machined ports clearly visible 

 
We’ll quickly brush over the process for replacing the liners in a 2 stroke engine as it is quite a delicate and unique process. Firstly a billet of centrifugally spun cast iron is machined to the correct dimensions and bored to receive the piston. The old liner is pressed out of the cylinder and discarded. 

 Cast iron blank ready to be machined , and the cylinder after the removed liner has be discarded.

The ‘new’ liner is pressed in either by cooling the liner or heating the cylinder.

This new liner isn’t ported yet. Each engine is slightly different, so the port layout needs to be transferred to the liner. This is done by pouring acid into each port. The acid will etch the cast iron, leaving a witness mark. The liner is then pressed out again ready for machining. 

Close-up of the etched liner.

The new liner is placed in the milling machine and the ports are machined into the liner wall. Great care must be taken to match up the angle of the ports to maintain the blend between port and liner.

 Machining of the liner.

The finished liner is then pressed into the cylinder, making sure that the ports line up perfectly during installation. Most liners have a flange machined at the top to retain the liners when in service. 

The installed liner with the flange clearly visable.

Wet or Dry
Cylinder cooling

Due to the nature of cylinder liners, some designers have gone step further and designed the liner to be the entire cylinder itself. This is called a ‘wet’ liner, as the block only provides the outside casing element. The liner is the entire cylinder, this increases the thermal conductivity of the sleeve as it comes in direct contact with the water jacket. A water jacket is basically the void in which the coolant flows. One disadvantage of using wet liners is the structural rigidity of the entire block.

Above is a 4 cylinder block which clearly shows the 'wet' liners installed.

The liners are fitted with either a flange seal, or ‘o’ ring. The liners have to be machined to either and some examples have both. The pictures below shows how these are fitted.

Dry liners just fit into a bored cylinder like the 2 stroke example and transfer their heat throught this material into the water jacket.

Engine Cylinder Design and Function - Part 5

Cylinder Material
Hard wearing, hard work



Cylinders have to ensure high load conditions and high pressure, as well as high temperatures. Even though the cylinder wall is lubricated, the cylinder still has to be constructed from a good bearing material. There are to main types of cylinders, sleeved and in block.

In the block cylinders are bored straight into the block, if the block is cast iron then the pistons can run directly in the block. If the block is Aluminium then the cylinder surface need a protective hard wearing surface. Some cast iron blocks also have coatings for that added protection and to increase the lifetime of the block.

 In-block bored cylinders - the block is cast iron an excellent bearing material

As in-block cylinders are self explanatory, we’ll look closer at coatings.

Aluminium isn’t a hard surface, and galls up quite quickly. One of the benefits is that Aluminium is a very good heat conductor, and with a cooler running engine the life of the cylinder is increased. However, the Aluminium wear issue is still a problem. This is where coating come to the rescue.

There are many coatings out there but there are 2 used more than any other. One coating is quite common, especially in the 2 stroke application. This is called Nikasil and is a ceramic coating that is very hard wearing and durable. 

Nikasil coating on the inside of a cylinder, 

notice the change in colour against the Aluminium.

The other is a very expensive coating primarily used in high end motor racing like Formula 1. This is called DLC which stands for Diamond-like carbon. Most modern razor blades use some form of DLC coating, usually Hydrogen free. 

 DLC coating on a cylinder sleeve

Both DLC and Nikasil coatings are usually applied to the cylinder wall through a process called physical vapour deposition (PVD), we won’t go into detail but basically the coating is evaporated and deposited onto the cylinder surface in a even smooth coating. These coatings protect the Aluminium underneath, increasing the longevity of the cylinder life.

One of the drawbacks to using these coatings is that wear will eventually prevail and the cylinder will slowly become oval in shape. The coating has to be removed before the cylinder can be re-bored and recoated. This is a difficult and expensive as the coating is very hard.



DLC coatings are becoming cheaper and so more popluar, they are used on both pistons and wrist pins.

DLC coating on the underside of a race piston.

DLC coating on Titanium wrist pins.

Engine Cylinder Design and Function - Part 4

Piston Slap
The problem with pistons


Piston slap, as previously mentioned is one of the inherent problems that have plagued engine designers for years and causes most of the wear in a cylinder. Increased wear in a cylinder reduces power output and consumes more oil. Before looking at the main issues caused by piston slap we need to understand what it is.

Piston slap is the as it sounds, the piston slapping or making contact with the cylinder wall. In a perfect world the piston is small in diameter than the cylinder and should therefore never maker contact with the cylinder. Unfortunately like most things in life, it isn’t as straight forward as that.

The piston not only moves parallel to the bore but also ‘rattles’ for lack of a better word in the cylinder. The piston travels in 2 directions which we’ll call up and down. On the downward stroke (in both 4 and 2 stroke engines) the piston is forced downwards by the expanding gas above it (the power stroke for 4 stroke engines only) When this happens the piston is tilted due to the outward movement of the connecting rod its attached to. Because the piston is attached to the connecting rod via a wrist pin the piston can slightly tilt around this axis. 

Due to the fact that the piston rings hold the top of the piston away from the cylinder (acting like a buffer) its the piston skirt makes contact with cylinder. The image above is a massive exaggeration, but shows how this happens. The black arrows show the crankshaft rotation, the orange arrow represents the pressure forcing the piston downwards. As you can see the piston rings would act as a buffer and the piston skirt to the left hand bottom of the piston makes contact with the cylinder. 

 
The same happens when the piston travels upwards towards the head, however the pressure against the cylinder wall is lower do to there no directly active force being applied to the piston. With this occuring thousands of times a minute at high rpm the cylinder begins to wear in an oval shape as show before. This oval wearing is always parallel with the wrist pins. 

This happens on both 2 and 4 stroke engine, however to a lesser degree with 4 stroke engines as they have better oil film layers and have 1/2 as many power strokes compared to a 2 stroke engine at the same rpm.


Once the piston has been at this for years of running the issue becomes worse. The more oval-ing the more the piston can tilt and the greater increase in wear rate. This leads to ill fitting rings and gas blow by becomes a bigger issue. Just to be clear, you might hear people say that their engine is suffering from piston slap and that an ‘slapping’ noise can be heard. All current piston engines suffer from piston slap (apart from big diesel 2 stroke engine but that’s for later) What they mean by this is that the wear in the cylinder has progressed to a point wear you can hear it.

So engineers have spent countless man hours trying to rectify this problem, or should I say reduce the effects of piston slap. There are a few designs that have been floating around for years now with some new ones appearing in the last 10-20 years. I’m not going to go over every single one as some are debatable to whether they actually work. So we’re going to look as some design features that have been used with some success.



Wrist pin location


One of the simplest methods of reducing slap is to shift the wrist pin location off centre. Wrist pins over the years have always been located in the centre of the cylinder for balance considerations and manufacturing simplicity.

 The image above isn’t the easiest to understand or illustrates the offset very well but it does show the tiny amount of offset required to make quite a difference, and the design work required to produce the desired effect.

What this achieves is to change the central pivot location of the piston, this in turn reduces the amount of tilt the piston can achieve on the power stroke (where most of the wear occurs) this does increase the amount of tilt on the upstroke, this however is where less of the wear occurs so its win – kinda win situation. As with most things in engine design, it’s a compromise.

Desaxe Engines
The French solution
 

Desaxe is French for ‘unbalanced’, the method is becoming more and more common. The Toyota Prius engine has desaxe cylinders. So what the hell is a desaxe cylinder? Normal cylinders, (if I can say that) are inline with the crankshaft pin. This means that the piston, con-rod and crankshaft centrelines are all inline, desaxe cylinders are slightly offset, this has the same benefits as the offset wrist pin without the inbalance the offset wrist pins. Its also recovers a tiny amount of force due to torque transfer, which we’ll cover in the crankshaft section. 

The image above shows this offset in relation to the piston etc.

As you can see from the picture above, when the piston is on the down stroke the angle of the con-rod is a lot stepper than it would be if the cylinder and piston were aligned. This reduces the thrust loading on the side of the cylinder.

Asymmetric Pistons
Aftermarket solution  



This all sounds good but what about engines that have already been built. Some aftermarket pistons have offset wrist pins, but this isn’t the only improvement that can be made. In conjunction with offsetting the wrist pin the piston skirt itself can be modified to help reduce the rate of wear.

This is achieved by increasing the surface area of the of the piston skirt, this spreads the thrust force against the cylinder wall, reducing the amount of wear compared to symmetrical piston.

 The image above shows the extra piston skirt surface area. The piston is out of balance, but with careful design the offset wrist pin moves the centre of gravity towards the centre of the piston.

Engine Cylinder Design and Function - Part 3

Cylinder wear


Either 2 or 4 stroke engines have 1 or multiple cylinders which have 2 functions 1 – house the pumping process of the engine and 2 – act as a guide for the piston. What a lot of people seem to ignore is that the cylinder is a bearing surface. Piston rings are one half of the bearing and the cylinder is the other.

Building of this you could think of the cylinder being the ‘outer race’ and the rings as the bearing material, but this is of little consequence. What matters if the first fundamental rule of cylinders, they need protection.

Protection comes in the form of an oil film, a thin coating of oil on the inside surface of the cylinder. Without this the rings would quickly heat up due to friction between the 2 and melt, yes melt!

Heat Seize

This is usually known as a heat sieze and can really ruin your day.

 
This tends to happen in 2 strokes more than 4 strokes due to the oil control and management of a 2 stroke design, but can and does happen with 4 stroke engines.

However there is something puzzling, why then do the piston show score marks? Has the piston come into contact with the cylinder wall? Does the piston touch the cylinder or not?

These are some good questions, so we’ll break them down to understand what’s going on. 

In an ideal world the piston shouldn’t come into contact with the cylinder wall, we won’t go into the function of the piston yet (that’s another page) but it shouldn’t. Unfortunately it does, however not at the top of the piston but at the bottom. The picture above shows wear marks at the top and not the bottom, so what’s going on?

Most of the heat that is transferred to the piston is through the piston crown (the top surface facing the head) All of the heat generated through friction in the rings also transfers to the piston. So most of the heat the piston encounters is in the top quarter of the piston. 

This causes the piston to grow (expand) and unfortunately this expansion is radial i.e outwards towards the cylinder. Hence when a heat seize occurs the piston usual collides with the cylinder and you get nasty score marks around the piston ring region.


Now with all that sorted lets go back to what we mentioned earlier. I stated that the piston makes contact with the cylinder at the bottom section of the piston. This region is called the piston skirt for obvious reasons. The piston can rattle around slightly in the cylinder as it pivots on the wrist (gudgeon) pin as the con-rod throws from side to side. 

This is unfortunately unavoidable with the current design and is something that designers have learned to live with. This action is called piston slap, and is the main factor of the majority of cylinder wear.

Larger stroke engines suffer more as a result of larger perpendicular (side-to-side) motion of the con- rod and thus the piston itself. This side to side slaping of the piston against the cylinder wall causes the cylinder to wear in an oval shape, and this oval-ing usually occurs further down in the cylinder, away from the head.