PISTON ANTI-DETONATION BAND TECH

A number of Cosworth pistons have what Cosworth refer to ‘piston anti-detonation bands’, but what actually are they? The term anti-detonation is perhaps a bit of a misnomer, as these bands don’t actually stop detonation. Instead, these bands mitigates the effects of detonation.

Piston Anti-detonation Bands


Understanding Detonation

Before we take a closer look at these bands, what actually is detonation? It’s a phenomena that is commonly mentioned in the world of high performance engines, but it does sometimes get used incorrectly. Detonation is broadly defined as unwanted or abnormal combustion. It occurs when both the temperature and the pressure in the unburnt mixture of air and fuel exceeds a critical level.

In a normal combustion event the spark plug will ignite the air/fuel mixture inside the combustion chamber. This happens before TDC whilst the piston is travelling upwards. The ignition of the air/fuel mixture creates a flame front that spreads outwards, igniting more of the mixture and leaving behind the burnt gases.

However, in certain circumstances when the temperature is hot enough, the unburnt mixture that is furthest from the spark plug (towards the wall of the cylinder) will ignite before the flame front reaches it. This unwanted and uncontrolled combustion can happen almost instantaneously, setting up strong pressure waves that hit the walls of the cylinder. These shock waves create the distinctive pinging noise that can be heard and indicate the presence of detonation.

Cosworth Anti-detonation Bands

In the early ‘90s, one step that Cosworth took to reduce the effects of detonation was to introduce anti-detonation bands. These grooves are located above on the top land above the top ring groove. They are typically around 0.2 mm deep and 1.25 mm wide. Most pistons will have two, three or four of these grooves, depending on the height of the top land.

But these shock waves can also be extremely destructive – they can inflict damage to the piston crown and in particular to the area around the top land. If detonation is severe then ultimately the piston will be irrevocably damaged, with the potential of engine seizure.

There are a number of Cosworth pistons that have anti-detonation bands, ranging from those for the YB through to some of the more recent Formula 1 engines. Basically, any engine that will experience high cylinder pressures will benefit from the usage of anti-detonation bands.

Cosworth YB Piston


The aim of these bands is to protect the top ring by disrupting any high-pressure pulse waves from detonation. The volume within these grooves, although small, allows a space for additional atomisation of the fuel and air mixture. When detonation occurs, this extra space provides an outlet for the shock waves.

These bands also bring about a couple of additional benefits. Firstly, they can prevent the build-up of carbon above the top ring which would otherwise cause the ring to stick in the ring groove. Secondly, they reduce the amount of contact between the top land the cylinder bore wall. This makes them very beneficial at high engine speeds and piston temperatures. Hence they are also occasionally referred to as “contact reduction grooves”.

The introduction of anti-detonation bands by Cosworth was another example of their experience and understanding of internal combustion engines. Indeed, when owned by Vickers, Cosworth actually patented their anti-detonation band design in the US.

Anti-detonation Band

Want to know more about the range of  Cosworth pistons that we supply? Then get in touch via our Contact Us page.

TIMING BELT TECH

One of the most important aspects of any IC engine is the correct timing of the rotational movement of the camshafts relative to the reciprocating movement of the pistons. Any errors in this timing will result in detrimental performance, and could in the worst case lead to contact between the valves and pistons, which could prove to be both catastrophic and costly. There are a number of ways that this meticulously choreographed movement of valves can be achieved, such as with gears or chains. But one of the most common methods in a road car is with the use of a timing (or cam drive) belt. The timing belt is normally driven by the crankshaft, and then turns pulleys that drive the camshafts.

Cosworth 20019488 YB Heavy Duty Timing Belt


Timing Belt Construction

The majority of automotive timing belts are constructed from an elastomer body that contains tension cords, with a fabric backing and a tooth jacket.

Looking at these four ingredients one by one, then the first is the elastomer body. This is normally a high temperature spec rubber such as HNBR (hydrogenated nitrile butadiene rubber) or EPD (ethylene propylene diene). HNBR is more suited to engines, as it can handle exposure to lubricating oils. The elastomer body can sometimes be reinforced with aramid fibres, which help strengthen the belt and provide extra protection for the teeth. Aramid is a heat-resistant and strong synthetic fibre that is sometimes referred to by the tradename Kevlar. Our heavy duty belts for the YB engine contain this aramid reinforcement, which helps make them three times stronger than a conventional automotive timing belt.

The tension cords help to give the belt incredible levels of tensile strength without compromising on flexibility. Tension cords are typically manufactured from high-strength glass fibre, and the individual strands of fibre are bundled together and twisted for added strength. It is the presence of glass fibre that means that a timing belt should never be ‘crimped’, which is the action of over-bending or twisting the belt that then shears the glass fibres.

Next up we have the tooth jacket, which is a temperature-resistant polymide fabric that helps to protect the teeth from abrasion as well as shear forces. The last ingredient is the fabric backing, which is usually another type of polymide fabric. This fabric backing is used on the smooth face of the belt that will run against the belt tensioner, so resistance to abrasion and wear is a must for the fabric.

Cosworth 20019488 Heavy Duty Timing Belt

Toothed Belts

In the majority of cases the timing belt is driven by teeth on the belt that engage with matching teeth in the pulleys on the crankshaft and camshafts. It is these teeth that provide the accuracy in timing, and hence toothed belt drives are often terms ‘synchronous belts’, as the keep the movement of the cams in synch with the crankshaft.

Keeping the teeth engaged at all times is vital. If a tooth jumps out of position then this is called ‘ratcheting’ (some people might use stronger language when this happens!). It is normal for the belt teeth to try to escape from the adjacent teeth in the pulley – when this happens the belt tension increases, which pushes the teeth back together, but this can lead to long lasting damage of the tension cords inside the belt. Complete slippage of the belt teeth out of the pulley teeth usually happens because either the tooth engagement is poor, or because of lack of belt tension.

The profile of the teeth is an important factor for the performance of the timing belt, and there are generally three categories for the profile – trapezoidal, curvilinear or modified curvilinear. Curvilinear profiled belts are sometimes called High Torque Drive (HTD), and modified curvilinear profiled belts are often called Super Torque Drive (STD), S-type Tooth Profile Dual-sided (STPD) or GT.

Belt Tooth Profiles

Tooth Profiles

The trapezoidal profile is the oldest of the three, introduced over 80 years ago. The shape of the profile is a trapezium, with straight flanks that are angled inwards towards the tip of the tooth. When the teeth revolve around the pulley, these flanks create an involute curve that matches the involute tooth profile on the pulley.

One disadvantage with the trapezoidal tool profile is that it has sharp corners at the root of the tooth, and this can create high stress concentrations that can weaken the belt. To overcome this, the curvilinear has fully radiused corners, which helps to even out the stresses. Also, the curvilinear tooth profile is taller than the trapezoidal profile, making it more difficult for the teeth to jump out of position and also giving a larger contact area, which in turn helps to reduce both stress and noise.

The last of these three profiles, modified curvilinear, is today the most popular type with timing belt manufacturers. As the name suggests, this profile is based on the curvilinear profile, but has a shallower tooth height along with an increase in flank angle. These changes help to give the modified curvilinear profile the ability to withstand higher amounts of torque.

Cosworth Engine Belts

Want more information on the belts that we stock for Cosworth engines? Then get in touch via our contacts page.

PISTON OFFSET PIN TECH

Our range of Cosworth YB pistons feature an offset gudgeon pin, which helps to reduce piston skirt wear and engine noise. To understand why, we have to consider the loads that the piston will experience.

Cosworth YB Piston Drawing

When the piston moves up and down the bore the small end of the connecting rod will articulate forwards and backwards around the pin. This creates a loading on the piston that pushes the piston sideways. This side load varies with crank angle and is also different for each stage of the four-stroke cycle. The largest side load occurs during the power stroke, when there is a combination of inertia and gas loads that will push the piston sideways towards what is referred to as the major thrust side.

Side Loads

These side loads can have an extremely detrimental effect on the operation of the piston. For example, they can promote tilting or rocking of the piston in the bore, which in turn can increase wear at the top of the lands and at the bottom of the skirt. This type of movement can also create vibrations that are then radiated through the engine and can be heard outside the engine.

One way to reduce the major thrust side load is to offset the pin’s centre away from the cylinder bore centreline and towards the major thrust side. Even just a small offset can have a noticeable effect in reducing wear and noise during running.

If you’d like to know more about the science behind piston offset pin tech, or about the YB pistons that Cosworth makes exclusively for Modatek, then please get in touch via our Contact page.

PISTON ACCUMULATOR GROOVE TECH

A piston accumulator groove is one way that the sealing of the piston rings can be improved. But why is ring sealing so important?

Piston Accumulator Groove

At the centre of any internal combustion engine lies it’s beating heart. Pistons travel up and down the bore at immense speeds, sucking and expelling combustion gases in and out of the combustion chamber. The piston has to withstand the immense loads from inertia and gas pressure. But they also have to provide a stable housing for the sealing rings to function correctly.

The sealing rings have to stop the combustion gases from escaping from the combustion chamber and into the crankcase. They also must prevent oil from travelling in the opposite direction, from the crankcase into the combustion chamber. So its vital that the piston is designed to keep the rings in place at all times.

Good ring groove design is of course important, but there is another design trick that piston manufacturers can employ is with the usage of what is termed an accumulator groove – you’ll see one in our Cosworth YB piston for example.

Accumulator groove

Look closely between the top and second compression ring grooves and you’ll notice a very small groove. This groove is called an accumulator groove, and its job is to help to control the movement of the top ring.

Cosworth PA2062 Piston Design Features

It is inevitable that some combustion gases will escape past the top compression ring. This might be due to oversize ring gaps. Another reason is the unsettling of the top ring when the piston rapidly changes direction at TDC at high engine speeds. To combat this, the small additional volume in the accumulator groove reduces the pressure on the underside of the top ring.

This reduction in pressure is an effective method to reduce or eliminate unwanted ring flutter. This is a phenomenon that can have an adverse effect on engine performance and can potentially lead to high levels of blow-by. (Blow-by is the escape of combustion gases past the ring).

The reduction in pressure in the accumulator groove also has a beneficial reduction in pressure above the second compression ring. Stabilising both compression rings will improve the sealing performance of the rings.


Want to know more about the range of  Cosworth pistons that we supply? Then get in touch via our Contact Us page.

HEAD GASKET TECH

Cosworth YB1429 Head Gasket

Multi layer steel (MLS) head gaskets, like our popular Cosworth YB1429 head gasket pictured here, comprise of a number of layers of thin sheets of spring or carbon steel that sandwich the sealing material.

Cosworth YB1429 Head Gasket

The upper and lower steel sheets contain a pressed beading. This beading runs around the perimeter of the bores and passageways to increase the local sealing capability. When the head fasteners are tightened, the beading deforms to create extra sealing pressure. This pressure ensures that the combustion gases, lubricating oil and coolant that transfer between head and block all stay away from each other and remain inside the engine. Ideally the deformation will be mostly elastic, so that the beading can adapt to changes in temperature and load. Most MLS gaskets including ours also contain an inner layer called a stopper that prevents plastic deformation of the beading.

Further, the top and bottom layers have an elastomer coating to reduce friction. This can help to prevent microscopic movement of the gasket. Without this coating, there could be damage to the gasket and the firefaces on the head and block.

Our YB1429 MLS head gasket was designed by the engineers at Cosworth and Victor Reinz. They wanted to create a superior gasket. They wanted one that would provide excellent sealing between the block and head for all of the high performance applications that the YB engine ends up in.

If you’d like to know more or wish to order one then take a look in our on-line shop.