Vehicle Design: Piston Design 101

This article provides a truly comprehensive look at the design considerations for pistons. Further articles are available which provide a look at piston rings, the connecting rod and many other components. Select the '101 guides' or 'vehicle design' category in the right hand sidebar lower down the page. Anyway onto the article:

Fig 1: Labelled image of a piston and con-rod.

The two main requirements of the piston are as follows:

1. Contain all the fluids above and below the piston assembly during the cycle.
2. Transfer the work done in combustion to the connecting rod with minimal mechanical and thermodynamic losses.

They key thing to note about piston design for absolutely any application regardless of the field is that it is HUGELY dependent upon the task that the engine will be required to fulfill.  The pistons for a 1000 horse-power racing car will be designed differently and made from different materials than pistons that are used inside a tractor for example. The shape of the piston crown (see diagram above) can also vary dependent on the type of engine you are dealing with as they can be used to change flow characteristics within the cylinder. The crown of a piston in a diesel engine is likely to look different to the piston of a petrol engine, especially in high performance applications.  Despite this all pistons need to obey the following;

Five Key Properties of a Piston:

1. Sufficient thermal conductivity
2. Low thermal expansion
3. High hot strength
4. High strength to weight ratio
5. High resistance to surface abrasion

A few definitions for the terms that are mentioned above:

Thermal conductivity: The ability of a material to absorb heat without causing damage or significant change to the materials microstructure or properties.

Thermal expansion: How much the material will expand when heated.

Hot strength: Ability to withstand stress, strain and shear at temperatures higher than room temperature. (Piston temperatures will be covered later in this article).

Strength-to-weight ratio: Fairly obvious this one, ideally we want a very light material with very high strength, the lighter and stronger the better.

Surface abrasion: The rate at which material wears due to rubbing on the surfaces of it, surface treatments can be added to metals to increase the surface toughness and make them more resistant to this.

The piston is obviously one of THE key components in any engine; it provides the seal, which enables power to transfer to the crankshaft so an effective design is key.

Heat is one of the biggest problems faced when designing a piston, an example of typical road car piston temperatures and their distribution is shown below:

Fig 2: Typical temperature map for a piston

So n the crown of the piston the temperatures tend towards around 250 – 300 degrees Celsius and gradually decrease the further from the combustion you go. The second key property I mentioned earlier was that pistons should have a low thermal expansion; this is because at these temperatures pistons will expand, especially on the crown and the top of the skirt. Because of this, pistons are actually tapered, although it is not obvious with the naked eye pistons are wider at the bottom of the skirt than the top to allow for expansion at the crown. If this was not done then when the piston expanded then it could potentially become too wide to fit within the cylinder.

Piston temperatures also vary with speed, which is key to note for higher performance engines, since at 1500 rpm a piston crown can be around 100 degrees Celsius cooler than when the engine is providing 5000 rpm. Piston spray jets may be added to engine in these high performance engines in order to lower the temperature of the crown. These feed of the main oil gallery and usually provide between one half and one litre per minute.

Piston Materials:                   PROS                                       CONS

Cast Iron                                Hot strength                          Mass

                                                Hardness                               Thermal conductivity

                                                Thermal expansion

Aluminium Alloys                  Mass                                       Thermal expansion

                                                Strength-to-weight                Hot strength

                                                Thermal conductivity            Hardness

Carbon Fibre Reinforced     Mass                                        Cost

Carbon (CFRC) -                    Strength-to-weight                Operating issues

(Research use only)              Hot strength

                                                Thermal expansion

The operating issues mentioned with the CFRC pistons include hydrocarbon emissions, since the material used for the piston has been observed to absorb fuel particles and then release them in the exhaust causing unwanted pollutants, this is also a knock problem with this technology. However it must be pointed out these pistons are purely experimental at the moment and engineers are experimenting with materials in an attempt to come up with better compounds. 

Manufacturing Effects:

To demonstrate the effects of manufacturing effects on the performance of a piston we will focus on only aluminium pistons, however similar effects follow across most metals.

For spark injection engines, cast aluminium can provide an intricate part at relatively low cost and low weight. Forged aluminium however can provide a finer microstructure and therefore higher strength when compared to casting, albeit more expensive.

Due to the additional pressures occurring in a diesel engine (since the fuel is self igniting) the pistons require local reinforcements. For example, a cast aluminium piston for a diesel engine would be made from a higher temperature alloy than for a petrol engine, and the following reinforcements may be applied:

• Refined casting methods; finer microstructure around the bowl (crown).
• Insertion of high strength cast iron piston ring carriers.
• Insertion of bushes into the pin bores.

That is the end of the main piston design article, further articles going in depth into the design of piston rings, connecting rods and loads of other major components are available in the '101 guides' and 'vehicle design' categories found in the right hand side bar. Thank you for reading and please feel free to comment any questions about this subject or any other topic you'd like to see me write a guide to. 

By Adam Feneley,

Brunel University

MEng Motorsport Engineering (Level 2)

Affiliate of the Institute of Mechanical Engineers

April 2011

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Steering System Design Considerations

Steering is one of the fundamental principals which you need to have a firm grasp of in order to design any vehicle. It can be an exceptionally complex system with large amounts of variation between systems. There are numerous tricks you can play by altering angles to solve issues with steering, and this article aims to explain in basic terms, the ways in which you can rectify some common steering problems when designing a steering system for an automobile or kart.

Content will be updated very soon as I plough through my research for the steering design for my formula zero kart feasibility study!

Figure 1: Toe Angle

Figure 2: Castor Angle

Figure 3: Camber Angle

Figure 4: King pin inclination

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How to Change Your Shock Absorbers

Shock absorbers are there to ensure handling ride comfort in cars. They are effectively dampers which lessen the effect of the bounce experienced when a car hits a lump or object in the road, it is and important part of the automotive system; when they become worn the lack of damping can damage some of the components in the car. It can also lower the MPG rate and efficiency of your car, costing you a fair bit in petrol in the long run, not to mention the extra adverse effects to our enviroment. Luckily, the process for changing them is fairly simple for a relatively knowledgable person and is exactly the same for left and right shock absorbers, so here is a simple step by step guide:

1. Prepare the car:
Park your car on a level surface and jack up the car, make sure you place the jacks against the frame of the car and not the suspension system, prolonged periods of holding the cars weight can damage the system. Ensure you place wedges behind your rear wheels to prevent the car from moving, loosen you wheel nuts and remove the wheel.

2. Removing the old shock absorbers
locate the lower attachment bolts and or screws of the shock absorber (consult your car handbook if in doubt to which ones), Remove them whilst clamping the shock absorber piston rod in a vice grip. Using a wrench or other tool, remove the upper attachment bolts/screws, your shocks are now removed from the car, remove it carefully.

3. Replace worn parts (bushings and grommets)
The key things to replace is the grommets and rubber bushings. Get the first grommet and put it concave side down onto the shock absorber piston rod. Then, slide the rubber bushing onto the piston rod, moving it down so it is in contact with the convex side of the grommet. Put a second grommet onto the shock absorber piston rod, sliding it down (convex side down) so it sits snuggly to the bushing. If there is a plastic strip around the shock absorber, remove it.

4.Installing the new shock absorber
Place your new shock between the upper and lower suspension arms, pushing the shock absorber piston rod through the hole in the upper suspension arm, tighten the upper fixing nut. Compress the shock slowly, until it lines up with the lower mounting holes on the shock and the suspension arm. Insert the lower bolt through the bottom mounting holes and the bottom of the shock absorber and tighten.

That's it!
Now all you have to do is replace your wheel and lower the car from its jacks!

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How to Repair a Cracked or Chipped Auto Windshield

The tiniest pebble or piece of grit flicked up off the Tyre of a car in front of you can fly at high speed into your windscreen and will most likely cause it to chip or, if you are really unlucky.... A crack!

In the winter this can spell big problems, even the tiniest chip, the car gets very cold at night, causing the glass to contract and when you start up the car and blast on the heaters it expands very quickly. This strangely is a technique used to strengthen glass but if an imperfection is present it causes it to get bigger, and very soon your chip can stretch out and become a crack and you will need a whole new windscreen.

If you have a fully comprehensive car insurance scheme then your solution for this is very simple. Autoglass offer a windscreen repair service, which is free under most fully comprehensive car insurance policies. But, you need to catch the chip before it develops into a crack. The machine used by Autoglass basically injects a clear resin into the crack and compresses it will air pressure, forming a very smooth and unnoticeable seal on your windscreen on many occasions you cant even tell that it has been done.

However, if you do not have insurance that covers this or they cannot repair it with the tool (which has a fixed nozzle and can only do certain sized holes) then you can try and repair it yourself, but it can be quite expensive, not as costly as buying a new windscreen though.

Doing a simple Internet search you can find lots of windshield repair kits and there are a vast amount on auction sites like eBay, which are often worth an in-depth look. The prices range from about 100 GBP to around 2000 GBP depending on the type and quality of glass you are dealing with and the size of the crack, another good idea would be to search how-to sites to find tips and guides on actually repairing your windscreen and if you have an unusual or especially old model its probably worth looking for a more detailed account.

A few good sites to consider are:

Rock Chip Kits: (USA)
http://www.rockchipkits.c om/.

eBay: (Worldwide)
http://www.ebay.com

Windshield Doctor (USA)
http://www.windshielddoct or.com/

Delta Kits (USA)
http://www.deltakits.com/

Screen Care (UK)
http://www.screencare.co.u k/

There is also quite a good tutorial video I found on metacafe.com which could come in handy:

http://www.metacafe.com/ watch/642354/windshield_replac ement_kits/

But I warn you know, do not try this is you have any doubts with what you are doing, not only is it a waste of money paying for the kit, but if you ruin the entire glass panel its going to be fairly costly to replace.

If you windshield has developed a decent sized crack then I would very strongly advise that you contact a specialist, cracks over 3 inches are difficult to repair even for the most experienced of windshield technicians and many will turn you away or offer you a new pane of glass.

For me and you, repairing a chip in our windscreen or a even a small crack should be fairly simple, but you will need a steady hand and a good understanding of the kit you are using to get a good clean and flat finish to ensure you do not compromise the glass, leaving patches and edges is not good, you are still leaving weaknesses in the glass.

But do not expect your windshield repair to be perfect, the chances are that you will see a slight difference in the glass but the key is to make it as un-noticeable as possible. If you can stop a crack from spreading across the windscreen and obscuring the view of the driver you have the job done.

Small marks left after the job are common, but should be about the size of a pin head if you have done a good job, the view through the glass which you have treated may have a slight distortion to it, again, this is not a problem as long as it does not hinder the vision of the driver. It's a very good idea to practice on a worthless car before hand if you can.

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How to Diagnose and Fix Broken Electric Windows

It is almost a fitted standard in modern day cars, electric (motorized) windows are gradually becoming the industry standard. But with the simple addition of an electric motor brings another component to an already complex machine. Another part of you car which can break and how do you fix it when it does? Luckily, very easily:

Car windows are made of tempered glass; this means it has been made specifically for safety in a crash, and it will shatter instead of breaking. This produces thousands of tiny pieces, which can cut you, this is handy in a crash because large shards of glass can be the end of you, but when trying to repair something that contains glass it is something to be wary of. So that's the safety check done, your window motor is stuck, slow, sluggish to respond or making very strange and abnormally loud noises in operation, how do you fix it?

1) Firstly park your car somewhere quiet, like your garage, because you will need to listen carefully to the motor powering the window in order to diagnose the problem.

2) Turn your cars power circuits on (half turn of the key) but do not start the engine, you only need the displays to come on and be able to access the battery, having then engine off means you can hear what is going on inside much easier

3) Now before going to in depth check every powered window in the car, if a few are working, and one or more are not then the chances are that the fuses have gone inside the motor.

4) Now listen to the winding motor whilst pressing the button up and down, if you hear nothing then you can remove the switch panel, which can be done in most cars without messing about with the door panels, just check your handbook.

Window Regulator (Above)

5) Now get a multi-meter and try reading resistance (ohmmeter setting) and check for changes between the three positions of the switch; up, down and in the centre. If you see any change then the fuse is not the problem, if you get no readings, then it's your fuse!

6) Now if you know what you are doing then you can remove the door panel and check that all the wires are in the correct places and that the motor, switch and pigtail are all properly connected. If this is all looking good then you are going to have to hand over to a technician, make sure you put everything back like it was first though!

If you have tried all of this then you are going to have to purchase a new motor ninety nine percent of the time. You can get after Market motors

fairly cheaply but OEM models are better, you get what you pay for in effect. You can buy with the regulator attached and if you do, do not try and take it apart the unit has to be installed as one piece.

If the door glass motor is the problem it will need to be replaced. After-market motors are cheaper; they don't last as long as OEM, and they are usually weaker. Some motors can be bought individually, but others come as an assembly with the new regulator already attached. If you purchase a motor/regulator assembly, DO NOT! Disassemble them. You will need to install the whole assembly as a unit. Generally this will be a cable-actuated regulator, and they are notorious for early failure anyway.

You can get various types of regulators including cable actuated, single arm, belt driven, and scissor arm styles. If you need to work on these be very careful and if you are using a scissor arm of single arm regulator I would advise highly against it as the mechanism used includes a powerful spring, which acts as a counter balance, and can cause very nasty injuries like severed limbs if you don't know what you are doing.

To a basic level windows are fairly simple to repair, but if you have any further problems id strongly advise a mechanic if not, make sure you have a decent mechanic mind and some technical knowledge before trying anything new.

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How To Bleed ABS (Anti-Lock Brakes)

If you have any problems with your break systems, like callipers or wheel cylinders, then the brake systems must be bled to remove the air to ensure that the brake pedal remains firm. If you get air trapped in you system braking becomes less solid and pressure to the pedal will provide soft braking, which can be quite dangerous, increasing the cars stopping distance. Firstly ALWAYS read your cars handbook and whatever information you can get about it before trying! a lot of good car repair and service handbooks can be found at charity shops and second hand book stores, well worth it.

Good diagram of location and basic workings of the braking systems of a standard car

Bleeding is a fairly simple operation on most vehicles, assuming no air bubbles have found there way into the ABS modulator assembly. The air in the system can be compressed, so when brakes are applied, the air must be compressed first before the liquid can slow the car, hence the feeling of soft brakes.

Tool exist especially to bleed braking systems, and it can be done fairly simply with a power bleeder, injecting tool or vacuum bleeder. All of these systems are equally effective, the idea is to flush the air out of the systems by pumping them with fluid.

So where do you start? The common place to begin bleeding your brakes is to begin with the ones furthest from the master cylinder, then the set on the same hydraulic circuit. You then follow this ending with the closest to the master cylinder. These solutions are fine, assuming you either just want to bleed you brakes to remove air bubbles, or you need to replace any of the system components which come after the modulator; calipers, wheel cylinders, brake lines or brake hoses. But if you need to deal with a component which is upstream from the modulator then the operation is slightly more difficult, although the above method will work, it is not 100% effective on the parts before (and including) the modulator.

The modulator has a lot of little places which air can become trapped in small cracks, and flushing it out has no effect, to solve this, you will need to consult a mechanic from a garage which specialises in your car, or find out exactly how to do it for your exact model, as the systems can be very different from car to car.

The modulator can contain 8 - 10 Anti-lock or traction control valves, aswell as other dead end ports and check valves. Some cars have special bleed screws built into the system which make this whole process a lot easier, allowing the air out of all the trapped areas. Others do not have bleed screws and needs a scan tool, which can cycle through the valve system whilst you bleed the mechanism, allowing all of the air to escape

If you are not experienced with cars and mechanics, do not try this! you can make it worse and then end up spending alot more money than you previously had to with a qualified mechanic. The best advice i can give you is to always read the book on your cars systems before trying ANY repairs yourself!

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How to Fix Window Wiper Fluid Leaks

How to fix windshield wiper fluid jets and check for leaks!

Window wipers - vital in modern motoring

It can be very irritating and often dangerous when your wind shield washer jets break, especially in cold weather, without a clean wind screen you can have a job seeing, especially in bright sunlight. So what could cause them to break?

If your wind shield fluid jets aren't working properly the chances are you have a leak in the tank, there are very few things which can go wrong with the actual system because it is so simple. A good place to start is not over looking the obvious. Check your tank, does it have fluid in? if it doesn't and you have filled it recently there is obviously a leak somewhere, in cold weather the plastic of both the tank and the tubes which carry the fluid can contract and crack. If you have water still there and your jets still do not work then you have a slightly different problem but i shall explain non the less. For a good video of checking for leaks, take a look here.

Unfortunately if this is the case you have little chance of creating your own water tight seal again so you will be better off buying new tubes or a new tank and fitting it yourself, or having a mechanic do it. But this isn't the only reason you washer jets don't work, if there is water in the tank and it still doesn't work then you can try this:

Unplug the tube from the jets and press the button, if water comes out of the pipe, then your jets are clogged, you can clean them with wire or wire brush to allow the water back through, or you can blast them with compressed air. At this point It would probably be a good idea to remove the reservoir and give it a good clean and remove anything which could cause it to get clogged up again.

If water didn't come out of the tube when you pressed the button you may have a pressure problem. You can use a test light to make sure that the the switch is sending a signal to the reservoir pump, if this doesn't work you can change the fuse. If the problem still persists then im afraid it is the job for a mechanic as the trouble lies with the electronics of the pump.

FINALLY: a few tips. Make sure that when you are pressing the switch that your ignition is ON! The pump is powered by the battery and wont work if you have the car turned off. The most common place on a car for leaks is the barber plastic fittings on the tubing, to source the leak just follow the tubes from the jets to the resevoir, checking for them as you go, especially at joins in the tubing.

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