Archive for August, 2006

The basic principle behind the petrol engine and for that matter, the diesel engine, is one of internal combustion, although the two differ in their respective fuel systems and their method of initiating the combustion process.

With the petrol engine used in such makes as Alfa Romeo, introduce an explosive mixture (petrol vapour and air) into a virtually sealed cylinder, compress it by moving one end of the cylinder toward the other then ignite the mixture with a spark. The resulting very rapid burn will force the moveable end of the cylinder (the piston) away from the fixed end (the cylinder head) and via a connecting rod and crank arrangement rotation will result.

For the above scenario to take place effectively other conditions must be present both before and after the burn (combustion). The process starts with (1) a piston moving downward within a cylinder drawing in a charge of petrol vapour and air past an open inlet valve which then closes, at the bottom of this downward stroke the piston, via the connecting rod/crank arrangement, will change direction and start to move up. As the piston rises (2), the fuel/air mixture will be compressed and in doing so will increase in temperature, just before the top of the upward travel a spark will be introduced into this now highly volatile mixture and it will start to burn very rapidly, by this time the piston will have past the top of its travel and is now (3) being forced down by the expanding gases of combustion. The piston again reaches the bottom of the down stroke and (4) starts to rise pushing out the burnt gases through the now open exhaust valve which will close just before the top of the piston travel. Just past the top of piston travel the inlet valve opens ready for the process to be repeated. This is the four stroke cycle explained and it is referred to in engineering circles as induction, compression, power, exhaust; it is also known by the shorter version; suck, squeeze, bang, blow!

Obviously this process is now duplicated across all cylinders with each cylinder carrying out a different task in the cycle relative to its neighbour. This phasing is achieved by crank and valve position and will ensure an even power output. As a general rule, the more cylinders an engine has the smoother its power output will be, this is mainly due to the shorter time between each power stroke. However, there will still be the situation involving Newton’s third law which involves equal and opposite reactions to any action, to counteract, or at least minimise its effect, the crankshaft has strategically placed counter weights incorporated into its casting. This has a very pronounced effect in evening out the severe direction changes experienced by the piston/connecting rod/crankshaft assemblies.

Mentioned above is the inlet/exhaust valve arrangement and its relevance to the correct operation of the engine. It is vital that these valves are opened and closed at exactly the right time for the engine to “breathe� properly. To achieve this a camshaft is mounted such that its rotation permits the valve operation relative to the piston position. To achieve this a drive arrangement is incorporated in the engine design so that as the crank turns drive is transmitted via a chain or toothed belt to the camshaft. It is most important that the drive between the crank and cam is correctly set at production and at replacement intervals during the life of the engine, this is usually achieved by the use of specific marks on the engine allowing the correct positioning of the shafts relative to each other before the drive is fitted. Should the timing of piston and valve come adrift there is a distinct possibility that the two will meet catastrophically resulting in a very large repair bill.

There are, of course, many other systems and components which enable the engine to run and these systems will be dealt with in later articles.

As engine power is transmitted through the gearbox, the gears themselves will have a tendency to stay in mesh making gear changing very difficult. Even more difficult, once a gear has been disengaged, is the selection of another. The difficulty arises from the fact that the speed of the two gears being introduced to each other is very different. To make this gear changing process smoother and more gradual a device is required that will interrupt the engine output before it reaches the gearbox internals. This device is the clutch.

The most commonly used clutch today is the single plate diaphragm spring type. Put simply, a circular plate (driven plate) with a splined centre boss and a ring of friction material fixed to either side is slid onto the input shaft of the gearbox, one side of this plate will press against the working surface of the engine flywheel which is, in turn, bolted to the engine crankshaft, the other friction lined side of the driven plate will be pressed against another working surface mounted onto an assembly called the pressure plate. With the clutch in the at rest position this friction sandwich is clamped firmly between the flywheel and the pressure plate and will, therefore, rotate at the same speed as the engine. As the driven plate is splined it can move back and forth a small amount when required. At the centre of the pressure plate is a circular spring steel diaphragm which is slightly conical in the at rest position. Pushing on the centre of the diaphragm, flattening it out, will have the effect of reducing the clamp effect produced. The last part of the clutch assembly is the release bearing which is mounted around a tube over the input shaft being able to slide quite freely and in turn indirectly attached to the clutch pedal via a cable or a system of hydraulics.

When the clutch pedal is depressed, the release bearing moves forward pressing against the diaphragm spring unclamping the driven plate and interrupting the power supplied to the gearbox, with the drive gone, selecting or changing gear is easily achieved. The gradual release of the clutch pedal by the driver once a gear is selected will reinstate the clamp force on the driven plate and power will once again pass to the gearbox and on to the driven wheels. Even with the engine running and the clutch not being operated, no vehicle movement will occur until a gear is selected.

Although various mechanisms are incorporated in the driven plate to facilitate smooth power take up, the main advantage of the friction material on the driven plate is that it will slip for a very short time greatly assisting this power transfer.

Various other permutations on this basic theme exist e.g.; paddle clutches, multi-plate clutches, electro-mechanical clutches, damped flywheel clutches, centrifugal clutch, etc; but they all exist to interrupt engine power to the gearbox so that gears can be selected and the vehicle moved away smoothly.

We have reciently been working on reviews on the different cars in the Alfa Romeo range. The revews give details on the look of the cars as well as some security details and reliability. Own an Alfa Romeo or perhaps you wish to know how your car fared or you just wish to buy one the reviews can be seen in the Alfa Romeo section click here to go directly to the Alfa Romeo main page.

If you wish to give your views of the cars or about the reviews you can have your say in the forum at http://www.carsparefinder.co.uk/forum/viewforum.php?f=63&sid=2bad05e4b3f86d5f044651b5c736e480.

Engine power in the form of rotation is delivered to the final drive from the gearbox. The gearbox is normally in the form of a cast metal casing which not only contains the individual gears, the selector mechanisms and the means of securing all the bearings necessary to allow the gears and accompanying shafts to rotate freely but also a reservoir for the lubricating oil vital to ensure a long service life and smooth, quiet operation.

Why do we need a gearbox? Without going into the depths of gear ratios and the like, which may be the subject of a later article, imagine jumping on a bicycle and trying to pedal away in the highest gear, huge amounts of effort are required just to keep moving without falling off and acceleration is extremely difficult. With the huge effort comes excessive strain on the entire drivetrain; pedals, chainrings, chain, rear block and the rear wheel assembly, all in all very inefficient. Now repeat the exercise starting off in the lowest gear, moving away and accelerating becomes easy no longer requiring the huge amounts of lung and leg effort. The whole outfit also becomes much more stable and further acceleration through the gears increases the speed far more comfortably without loading the components beyond their normal limits. Now instead of accelerating through the gears see the rider staying in the lowest gear, acceleration and top speed will be limited by the speed the rider can spin his legs and there is a definite limit to this. Using a selection of gears permits the progressive acceleration up to the required speed and the facility to choose a gear to suit prevailing road conditions.

The effects felt by the rider are very much like the effects experienced by a motor vehicle when used without a selection of gear ratios being available. The high gear scenario will require the extensive slipping of the clutch, greatly shortening its service life, and the general fuel efficiency will suffer. A vehicle fixed in a low gear will obviously move away from rest with no problem but further acceleration and the maximum speed will be greatly limited by the noise from the engine just prior to it failing in a most catastrophic and expensive manner!

The specification of gear ratios and the number eventually offered by the gearbox will largely depend on the vehicles intended use. This is the reason for large commercial vehicles having, in most cases, twelve to sixteen forward gears. The requirement to overcome initial resistance to movement caused by a much larger vehicle coupled with a sizeable load means that the laden truck must have a greater number of low ratio gears. This also applies throughout the acceleration process with a larger number of intermediate gears being required before reaching the higher ratios and the desired final speed. As this process is a much more drawn out affair in a commercial vehicle the driver will endeavour to make the most efficient use of engine power, this is usually achieved by taking much greater notice of engine speed and therefore making greater use of the power range indicated on the dashboard engine rev counter, the so-called “green band�.

A much smaller vehicle will have no such requirement even if it is asked to tow a trailer or caravan of an approved weight, so the provision of four, five or even six forward gears will suffice.

To summarize, the gearbox is fitted to provide a range of ratios allowing easier acceleration and the maintenance of a required road speed without either under or over revving the engine thus promoting greater fuel efficiency.

One last, but very important feature, the gearbox offers the facility to go backwards!

From the previous articles there now exists a small collection of tyre/wheels isolated from a bodyshell/chassis and that in turn can be steered and has a dual circuit service brake system with a separate mechanical park brake.

Depending on the vehicle type, either two or four wheel drive, engine power has to be fed through the gearbox and reach the driven wheels; this is the job of the final drive. The component list will vary from model to model but will usually be drawn from the following; live axle(s), driveshafts, propeller shafts and differential. Common to the vast majority of exposed transmission shafts is the requirement for the shaft to be able to accommodate the movement of the wheel/axle assembly; this is achieved by the incorporation of universal joints of one kind or another at either end.

Taking probably the most common vehicle type, the front engined front wheel drive car, rotation is taken from the engine, through the gearbox, transmitting directly to the integral differential unit which will turn gearbox output through right angles also allowing each driven wheel to turn independently. Drive then exits the differential via a driveshaft to each side. With the front engine rear wheel drive arrangement gearbox output is passed directly to a propeller shaft, which carries rotation to the rear mounted differential unit. Should the vehicle have a solid live axle the differential passes the drive to either wheel via half shafts encased within the axle, with an independent rear suspension system the output is carried to the drive wheels by exposed driveshafts which, as already mentioned, have universal joints allowing suspension movement. Moving on to four-wheel drive vehicles, power needs to be distributed to all the wheels and to achieve this a transfer unit is mounted at the end of the gearbox with two shafts exiting, one to the front and one to the rear. The shaft arrangement is virtually the same as either of the two systems above depending on whether the vehicle has all round independent suspension or not. As with the differential at either end of the vehicle allowing either side wheel to turn independently, four wheel drive vehicles have a centre differential to enable the front and rear axles to turn at different speeds, an addition to this is the diff lock which ensures that power distribution is evenly split front to rear when traversing slippery terrain thus retaining grip and allowing progress to be maintained. This feature is normally available to the driver as a lever mounted next to the normal gear lever or as a switch mounted in the dash area.

As a brief aside, many four wheel drive vehicles have a selector arrangement which allows the drive to the front axle to be disconnected at the whim of the driver, this disconnection will have the effect of reducing fuel consumption and tyre wear.

The principles and construction behind transmission shafts is simple enough, a bar or tube with a splined or bolt fixing at either end. The decision to specify bar or tube largely depends on the length and the power to be handled, a long propeller shaft is better made from tube so as to keep vehicle weight under control, a short driveshaft maybe measuring around half a metre is probably easier produced from a bar. By and large a correctly designed and manufactured tubular shaft is capable of handling all vehicle requirements and has the added advantage of less weight.

Lastly, what is a differential? As well as providing the vehicle manufacturer with another avenue to specify drivetrain gearing, the differential is absolutely vital in allowing the vehicle to turn left and right. Imagine a car travelling clockwise in a fixed diameter circle, if you were to measure the circumference scribed by the left hand (outer) wheels you would find that distance greater than the distance travelled by the right hand (inner) wheels. As all the wheels are fixed to one body (the car) the outer wheels must be allowed to turn faster or the car would be forced to travel straight ahead. Forcing any deviation left or right from the straight on via steering input would result in massive tyre wear, transmission damage and a huge increase in fuel consumption not to mention making the vehicle virtually undriveable.

It must be fairly obvious what the braking system is for; it allows the vehicle to be slowed in varying degrees and also provides some method of holding a stationary vehicle without having to continually apply the service (foot) brake.

The braking system on any motor vehicle, whether it be the smallest glass fibre two seater city car or the fully laden heavy haulage tractor unit and trailer relies on the basic principle of friction. It must be mentioned at this point that the majority of heavy commercial vehicles and buses/coaches have supplementary braking systems which do not use friction, instead they have various devices/systems which have a direct effect on the engine or exhaust, alternatively a system utilizing the principles of electro-magnetism mounted into the drivetrain.

Returning to the average family car, two entirely separate systems are fitted; 1) the service or foot brake system and 2) the park or hand brake system. With the service brake, the brake pedal is connected to the brake master cylinder and when the pedal is pressed hydraulic pressure is applied through the rigid metal pipes and the flexible brake hoses to the brake cylinders or brake calipers mounted at each wheel.

The calipers/cylinders are absolutely no different in their principle or operation to the huge hydraulic rams seen on earthmoving machines or in the tipping gear of trucks it is only their appearance that is so different. Simply, hydraulic pressure exerted within a sealed space with a moveable section at one end (a piston) can be used to move or operate another mechanism, in the case of a brake system, a friction device. All that is needed is the friction device (a pad or shoe) to be pressed against a rotating body (a disc or drum) in turn attached to the wheel assembly, as the exerted pressure is increased so is the clamping effort on the disc or drum thus slowing its rotation.

Obviously the metal construction of the pad or shoe will need to be faced with a material which not only provides an effective degree of friction and a high resistance to heat, which is an unavoidable by-product of friction, but have excellent wear characteristics without producing premature wear in the disc or drum. The maximum amount of hydraulic pressure generated will depend not only on how hard the driver presses the brake pedal but also the relationship between the diameters of the master cylinder and the cylinders/calipers at each wheel.

One of the most important features of the modern service brake system is the dual circuit. With the old single circuit designs all the hydraulic lines were shared so that if a leak occurred all brake effort was lost. The dual system offers split circuits so that in the event of failure two brakes will work albeit with much greater pedal travel. The split will normally be arranged so that the two remaining brakes will be one at the front and one at the back usually diagonally opposed.

With the advent of the dual circuit system less emphasis is placed on the second brake system fitted to the motor vehicle, the park or hand brake. Now the car has effectively three brake systems, two almost separate hydraulic service and one completely separate mechanical only park. Regulations dictate that the park brake must be purely mechanical in operation, the most common being a centrally mounted lever in the passenger compartment coupled to the rear brake assemblies via a cable arrangement. Pulling on the handbrake tensions the cable(s) pulling levers inside the brake assembly applying the shoe or pad against the disc or drum. As the park brake must be able to be applied and locked in the on position, a ratchet mechanism is incorporated in the handbrake lever so the brake can be set only releasing when required by the driver.

Don’t let Gordon Brown win, play the new CarSpareFinder Fuel Tax Revolt game! 

The Government is toying with the idea of raising taxes on air travel and high-polluting cars in an attempt to remedy the UK’s spiralling greenhouse gas emissions. In addition, the 70pmh speed limit on Britain’s motorways could be slashed or more strictly enforced in order to reduce the amount of carbon dioxide given off by cars. 

But what does this mean for Britain’s motorists? Obviously, the cost of driving would increase substantially, with yet another hike in petrol prices, and owners of large, gas-guzzling cars will be hit especially hard with a massive rise in road tax on such vehicles. 

Clearly the Government is trying to steer people towards public transport or for those that must travel by car, into purchasing smaller vehicles with lower fuel consumption and emissions. 

These changes would not only have an impact upon the Government’s battle with climate change, but would go a long way to lowering congestion on Britain’s roads. The cost of travelling by car will be too much for many, and the lowered speed limits may make the train network a more desirable option for those short on time. 

You can’t argue with the motive behind these changes, something must be done to combat climate change sooner rather than later. But my major issue with the new plans are the way in which they will undoubtedly do much more harm less financially secure motorists. 

The wealthy businessman, in his state-of-the-art Jaguar or Range Rover will continue as ever before, the inflated costs a drop in the ocean to him. Meanwhile, the hard-up commuter will be forced onto public transport whether he likes it or not.

For me, this is going to create yet another divide between the rich and the poor in Britain. I use my car on a regular basis for both work and play. Each year, I clock up thousands of motorway miles simply because it’s much easier than travelling by train or coach. 

To use public transport in my case would be a nightmare, virtually impossible. But can I afford to pay up to £2,000 a year in road tax, on top of increased fuel prices and expensive insurance? Quite simply, I cannot. 

The Government have to act on climate change, I’m not debating that. However, I am not at all convinced that hitting motorists quite so hard in the pocket is the right way to go. With house prices at an all time high, the cost of living is simply proving too high for many. I fear that this financial shake up may well go some way to solving the environmental issues we face, but at the same time could all but destroy the economy. 

The powers that be need to think very, very carefully about what they are doing before running headstrong into any major decisions. 

On a more light-hearted note, why not play the new CarSpareFinder Fuel Tax Revolt game. The objective is simple. Prevent Gordon Brown from increasing fuel tax by electrocuting him in the face and testicles. If only it really were that easy… 

Every fitment in or on a motor vehicle has to be mounted to the bodywork or structure. By and large the bodywork performs various functions aside from the aesthetics; it provides a weather resistant environment for those inside and also facilitates the fitment of equipment to make that environment very comfortable indeed. Everything from the latest in multi-adjusting seats, through climate control, road noise suppression, a whole glut of airbags and the most up to date in satellite driven route finding not to mention mind blowing in car entertainment systems. From a purely engineering point of view all this equipment has to be securely fixed if it is to function as the manufacturer intended.

The forces generated by not only the weight of the vehicle itself plus occupants, their luggage, fuel and hundreds of items of ancillary equipment but also the side effects of the vehicle simply being driven along must be correctly calculated and distributed throughout the structure. This can only be achieved by producing that structure from a wide range of material specifications. This means close attention to not only the type of material used but also its dimensions and shape; what may be the ideal profile for one body panel will not necessarily mean that it would be correct for another.

The two main structure types are separate chassis and monocoque (sometimes referred to as integral construction). The monocoque, although probably the most common in car/light commercial production, has been in existence long before the internal combustion engine, take a look at the construction of the type of trailer pulled by a horse, the drawbar is fastened to the body, the body is either flat or adapted to carry a load requiring side panels and the rear may have a moveable tailboard for unloading, it has no separate chassis. The main reason for the success of this type is that it suits the modern production methods including the ubiquitous computer driven robot. The separate chassis construction is not as old as the integral type but does date back many years, its strength lies in the adaptability of a simple frame, make a powered frame with the essentials fitted, transmission, steering, brakes, etc and then bolt on the body which suits your particular needs, be it multi seating, flat back, tipper, tanker and so on. In the world of heavy commercial vehicles it is normal practice, when required, to cut a chassis and add in extra length for the fitting of a particular and/or unusual piece of equipment, this can include extra axles to spread the vehicle load.

Common to both structure types in the design and production of modern motor vehicles is the requirement to protect the occupants from crash injury. This is not done by simply making the whole vehicle a rolling bank vault, that would be prohibitively expensive and the result would be something akin to a main battle tank, complete with fuel consumption to match. The answer lies in the very careful engineering of certain areas e.g. the uprating of the mounting points for the seats and seatbelts, the fitting of side impact protection bars and the strengthening of the A, B and C pillars, it can also include the incorporation of “weak� areas, the much vaunted “crumple zones�. These zones are specifically designed to deform under impact folding and compressing thereby absorbing as much crash energy as possible before it reaches the driver and passengers. Very often, a car involved in a head on collision will display a completely destroyed frontal area but will have a relatively untouched passenger compartment.

There’s much debate about the pros and cons of the internet. The recent advertising campaign of one of the world’s most renowned ISP’s, AOL, was based around just that – the benefits the ‘net brings to modern life, and its polar dark side.
It would be naïve to suggest that the web doesn’t have its problems. It’s common knowledge that unscrupulous individuals do indeed occasionally exploit the web to their own advantage.  

Nevertheless, for every negative portrayal of the internet, there are a hundred success stories. The problem is, bad news is more saleable than good, and so the media is filled with tales of doom and gloom. The good stuff is resigned to the newspaper editors’ recycle bin, and never heard of again.

What is not documented often enough is the unprecedented convenience the web brings. You can now do your weekly shopping online, check your bank balance or book a cut-price holiday. There are so many web based services that go against the grain of today’s hectic modern society, making things quicker, easier and much less exhaustive.

And that’s exactly what we’re doing at CarSpareFinder. We provide a service which provides great value to two very different sets of customer; the consumer and the breaker. Each has very different needs, but the needs of both must be met in order to stage a successful business.

When we began working on CarSpareFinder, almost a year ago, we wanted to achieve two things.

1. To provide customers with the ability to quickly and easily locate spare parts (Even those that are notoriously difficult to find) and purchase them for the best price possible.
2. To provide car breakers with a steady flow of potential customers. In essence, doing their own marketing for them.

Ultimately, despite their differing requirements, the objective is the same for both consumers and breakers. The objective: to save time and money. It’s quite simple – we offer consumers a choice of quotes to choose from, and put the business on a place for breakers. And it couldn’t be easier.

It is, even if I say so myself, a wonderful service. That isn’t just my own opinion; it’s confirmed on a daily basis by all of the positive feedback we receive.

It’s taken off in a way even we couldn’t have predicted, but that’s the beauty of the internet. It allows passionate, hard working individuals like ourselves to make headway without being suffocated by the corporate machine. In just a few months, we’ve gone from inception to a highly successful, fast growing, innovative, online business.

The press need to take note. The internet offers a fantastic platform for enterprise and innovation. New, web-based ideas are constantly making the lives of the general public easier. And behind these ideas are genuine people, with genuine intentions offering a genuine service.

CarSpareFinder. Remember the name.

Apart from starting and stopping probably the most obvious function required of a motor vehicle is that it must be able to be directed in varying degrees to the left and to the right.

So what, other than changing direction, is required of the steering system? One of the most important is the transmission or “feel� from the steered wheels to the driver. This feedback will give a good indication of the condition of the road surface being driven on and also an indication of the state of the front tyres. Rolling over a frozen surface will produce far less noise and vibration than a good road surface; the steering will almost certainly feel much lighter and will become rather vague. A tyre operating at a reduced pressure or one that is almost flat will induce drag, make the steering feel unresponsive and cause the vehicle to wander. Correct interpretation of the above via the steering may well prevent an accident caused by loss of control.

Starting at the steering wheel, a component usually made up of a metal alloy structure covered with a trim which provides bulk and grip for the driver. Driver input is transmitted, via a shaft, through to the steering mechanism. The shaft itself not only has the task of rotating but also it must either collapse or move away from the driver in crash situation. This is normally achieved by a combination of joints and crushable sections. The added advantage of the inclusion of these joints is that they will allow the steering shaft to exit the passenger compartment at angles more easily connectable to the steering mechanism; the positioning of this mechanism may well be dictated to some extent by the design of the body shell and/or subframes.

There are six main types of steering mechanism; worm and sector, screw and nut, recirculating ball, cam and peg, worm and roller, rack and pinion. By far the most common system used on modern cars and light commercial vehicles is the rack and pinion. This system is simple in operation, lends itself easily to the addition of power assistance, is relatively lightweight and has little or no maintenance requirements other than routine inspection. All in all, the steering mechanism itself is low maintenance; however, it is important that regular inspections are made in the area of basic wheel alignment, commonly referred to as tracking. Faults in this area normally manifest themselves as irregular tyre wear and/or poor handling. On a less frequent front the owner may wish to have a more comprehensive steering/suspension check done of the castor and camber angles. All of the above factors will have an effect on one other desirable feature, that of self centreing. It will become very tiresome if the driver is forced to manually return the steering wheel to the straight ahead position after every turn, correct steering geometry is vital for this to happen.