John1uk

Well i didnt know anything until i read this..........


Rolling Road

I know what the rolling road is for but how does it work?

For four wheel drive cars it has two rows of rollers which support the car. The front roller of each set is connected to a huge electromagnet. Think of it like a massive disc brake but with magnets not brake pads.

With the car resting on the rollers and the engine running, 1st gear is selected, the clutch is engaged and the rollers start to turn. (The rear of the two rollers on each set measures wheel speed.) If at this point the accelerator is pressed and more power is transmitted through the tyres there is no resistance so the rollers accelerate rapidly. This is just like nailing the throttle in neutral.

So we can try to "brake" the power of the engine, hence the term Brake Horse Power or BHP. If we can match the braking force of the rollers to the power being transmitted through the tyres we will create a torque reaction in the electromagnetic brake (or brake caliper). The amount of energy required to balance the torque transmitted by the car is then measured. The computer then looks at torque output and revolutions of the wheel (recorded by the other roller) and displays power transmitted at the wheel. There is another set of roller output figures on four-wheel drive cars so both figures are added together. This gives us power at the wheels.

How do you come up with flywheel power then?

The test is carried out in fourth gear. (This is because 4th is usually direct on most cars so gives the least losses). We dial no load into the rollers and get the car rolling in 4th at 1000 rpm. Then the throttle is opened fully and resistance is applied to match engine power. As engine power increases (when the turbo comes in) the resistance is increased, just to provide a steady smooth rate of acceleration. When maximum power has been shown and power is decreasing the transmission is disengaged and allowed to free-wheel on the rollers. This means the huge amount of energy built up into the rollers is reversed and they drive the transmission up to the clutch.

There is then a negative torque reaction and the amount of power required to drive the transmission is added to the power previously measured at the wheels. Simple eh?

Err...kind of.

We are not claiming it is the world standard for measuring power but the German manufacturer of the equipment has been making chassis dynos since the war and it is fast, repeatable and accurate.

There is no tyre slip, because we can measure it if it happens, it is not hard on the engine, because the test only takes less than a minute and we have been doing it for years.

Give us a non-turbo car we have never seen before (turbo cars are affected much more by atmospheric conditions, heat build up and inaccurate boost settings) and we will give you a power output figure which we know will be within 1-2% of the manufacturers claimed power figures.

The only people who criticise rolling roads are tuning companies who don't have one. These companies know that they cannot do as good work as people with them so they say they are unsafe, inaccurate or dangerous.

Would we have spent more than £120k on two of them if they were useless?





Turbochargers

A turbocharger is a gas driven gas pump -- it utilises gases to pump around other gases.

It does not provide free power as has been claimed.

The idea is to use the exhaust gases to drive a compressor which compresses the air going into the engine -- making it denser, which means there are more oxygen molecules per unit of volume -- so it makes a bigger bang when ignited with more fuel. Hey presto! More power. One way of looking at it is that a turbocharger enables a 2 litre engine to burn as much air and fuel as a normally aspirated 3 litre engine, or even more.

Exhaust gas which is being pushed out of the cylinders is forced through a restriction and round a snail-shaped housing called the turbine housing. The snail shape causes a speed increase as the gas turns through 360°. At the centre of the turbine housing the gas spins a turbine wheel. This wheel is a high temperature steel alloy on 95% of all turbocharged cars, but can be a lightweight ceramic as used on the Nissan Skyline for example. This turbine wheel is connected via a shaft to the compressor side of the turbo. This is inside another housing called a compressor cover. The compressor's job is to suck in air through the air filter and accelerate it and compress it before forcing it into the engine. This creates a positive pressure (i.e. a pressure greater than atmospheric pressure which in it's natural state is 1 bar or 14.7 psi). However before the turbo spins up to speed the compressor acts as a slight hindrance to the flow of the air coming into the engine. The size and shape of the turbine housing, wheel, shaft and compressor cover dictate airflow in and out of the engine, and therefore power and response.

The shaft between the turbine and compressor runs in a bearing housing which is water cooled. There is not actually a bearing in the bearing housing but a guide channel through which oil passes. This pressurised oil becomes the bearing and takes up the clearance between the shaft and the housing. This is why when you feel the shaft movement on a turbo when the engine is not running it always feels as if the shaft bearing is worn out.

The bearing housing on the Impreza (and 98% of all turbo cars on the road today) is water cooled. Water cooling of the bearing housing is something developed in the '70s when oil was not as good as it is now. It was developed to keep the bearing housing cooler after engine shutdown and avoid oil burning on the shaft. It is still used today by all manufacturers but these days modern oils do not burn on the shaft even after hot shutdowns. But it would be a brave manufacturer who admitted they had fitted a non-water cooled turbo to their latest super, quad turbo, five valve, V12, tarmac eating stonkermobile.

If you are using the car extremely hard then just allow a few minutes for it to cool down or use a turbo timer. It is not vital but it helps.

So this is a turbocharger.

If bigger turbos make more power, I want a bigger one.

Bigger turbos also mean more lag.

What is lag?

Lag is the time taken for the exhaust gas to spin the shaft fast enough for the compressor wheel to start to generate some positive pressure. Bigger turbos take longer to spin up so there's more lag.

Great, now I want a smaller turbo.

Smaller turbos spin up faster so there's less lag but they flow less air so deliver less power.

Now I'm completely confused, I think I'll take the damn turbo off.

No don't do that they are great fun. It is just that the rules governing them are not straight forward. The game engine designers play involves balancing exhaust gas flow with compressor air flow volume. Add to this equation shaft speed, shaft weight, compressor and turbine housing flows and turbine wheel tip speeds and you can see how difficult things are. (And I haven't even mentioned inlet and exhaust cam design, exhaust primary and secondary pulsed extractor lengths, variable turbo inlets and wastegates!)

So what is a wastegate?

A turbocharger has a very limited shaft speed band during which it is efficient. But an engine works over a huge range of gas volume output (rev range to you sir).

Consider a diesel turbo for a truck. Most diesel turbos are matched to the engine flow. Therefore they get up to their efficient rpm and start pumping positive pressure. Then as engine revs rise so does boost pressure so roughly speaking max boost is at max revs. No problem on a diesel because they have such a limited rev range.

Take your Impreza. You want boost at low speed and boost at high rpm too. So the turbo is designed to spin up quickly and provide positive pressure at say 2500rpm. It will then make more boost until the turbo shaft reaches maximum speed at say 3500rpm. At that speed if the turbo goes any faster two things will happen very close together. Firstly the turbo will overspeed and start to explode (the tips of the blade will actually go supersonic and the resulting sound shock wave will start to break the tips off of the turbine wheel!), secondly it will generate rapidly rising boost of 2-3 bar (30-45 psi) at which point your engine explodes too.

Oh.

So instead a valve opens inside the turbo which allows exhaust gas to escape without going around the turbine housing. This slows the turbine wheel and reduces boost. Then the valve closes and more gas goes through the turbine housing therefore increasing boost until the valve opens again, etc, etc.

This wastegate valve is controlled by a pneumatic actuator (although the Ford Focus World Rally Car has a hydraulic one) which itself is controlled by an electric solenoid valve which is controlled by the ECU which is controlled by the software which can be re-programmed by Power Engineering. Clever eh?

So do you understand about turbochargers now?

Cornelius

iTrader: (2)

Nice one John, some good information there!!

Her at the back

Went i asked last time every laughed at me!

John1uk

trails

iTrader: (41)

explained so well even i understood itthat should be made into a sticky for the technical section...ta