Tekst na engleskom koji se bavi modifikacijama Una Turba. Dok sam vozio ovaj automobil ovaj tekst mi je bio koristan pa ga ću ga podeliti sa onima koje ta tema zanima.

Uno Turbo Modifications.

Introduction:

Before one can contemplate modifications to this engine you need to fully understand how a turbo-charged engine works.

The Air Engine.

Any piston driven engine is basically an air pump. In a conventional (non-turbocharged engine) the job of filling the cylinders in done by the pistons. On the inlet stroke the falling piston produce a partial vacuum in the inlet manifold and the fuel-air mixture is „sucked“ into the cylinder. On the compression stroke the fuel-air mixture is compressed and ignited by the spark plug so producing the energy to drive the piston down-wards. On the final stroke the pistons expel the burnt gases out of the cylinder into the exhaust system.

The Turbocharged engine.

The turbocharged engine is radically different to the conventional type, not so much in the mechanical parts, but in how the inlet fuel-air mixture (the charge) is ingested.

In a turbocharged engine the inlet manifold does not operate at a partial vacuum but is positively pressurised by the output from the turbo. You need to appreciate that most turbocharged engines use a comparitively low compression ratio. In high performance engines compression ratios of 14:1 or higher are used for a very good reason. Firstly by using a high compression ratio the vacuum produced in the inlet manifold is greater and a larger volume of charge is ingested into the cylinder. On the compression stroke the mixture is compressed to a pressure which is x number of times over the atmospheric pressure. (The compression ratio is simply the swept volume of the cylinder divided by the combustion chamber volume) For a turbocharge engine the swept volume is fixed, as in the conventional engine (non-aspirated) and so is the combustion chamber. But as the manifold pressure is now positive relative to the cylinder, more charge is ingested. In an example using the case where the compression ratio is say 7:1 and the boost from the turbocharger is 1 atmosphere (1 bar) the effective compression ratio is 2x the actual or 14:1. Power output is directly proportional to the size of the combustion „bangs“ and the revolutions per minute of the engine. For high output powers a combination of high boost and high rpm will give high output power. (Duckworths formula!)

If you have read this far and fully understood the above you are now in the position to understand the final part. If you didn’t understand the above – go back and read it again. Only when you fully understand the principles can you see how adjusting the boost pressure can give added horsepower.

Increasing the boost.

For a given amount of boost and a static compression ratio, for a particular type of fuel, there is a certain maximum output power that can be obtained.

By increasing the boost, assuming that no bad side affects occur, then the power output is directly proportional to the boost. By bad side affects I mean premature ignition (knocking or „pinking“) or failure of the rotating parts, eg a connecting rod

failure causing severe engine damage. All of these are possible in a highly stressed engine. The Uno Turbo is generally a very good mechanical model, but all items have their limit.

The Down-side of turbocharging.

In the conventional engine (non-turbocharged) the trick to getting the maximum horsepower is to keep the inlet charge as cold as possible. If this can be achieved the charge will be dense and will allow the maximum power to be extracted from the fuel-air mixture. Unfortunately with the advent of turbocharging, which incidentally came about during World War 2, the air passes through the turbocharger housing and due to the combined effect of compression and the proximity of the exhaust impeller housing is heated to a temperature well above the ambient. The inter-cooler job is to reduce temperature of the air to something around ambient. This will depend on several factors. Firstly the ambient temperature. Secondly the surface area of the intercooler should be as large as feasible. Thirdly the internal volume of the intercooler should be small. All of these are variables and often, as in most engineering problems, a compomise has to be made. A large surface area for the intercooler often means the internal volume is not as small as one would like. The penalty of this is that the turbocharger has to fill the internal volume with a lot of air before the boost pressure comes up, the so called „turbo lag“.

Turb lag is a fact of life and there is little one can do except adapt ones style of driving to the limiting factors. Having been used to driving a turbocharged vehicle if one now steps into a normal vehicle the driver has to readapt to prevent wheel spin on take off.

Increasing the density of the inlet charge.

For a particular atmospheric pressure and temperature there is a definite amount of fuel-air mixture which can be maintained without the fuel falling out of suspension and becoming droplets of petrol. A very good way to both increase the density and lower the charge temperature is to add water to the inlet mixture. Water is very low cost and has several plus factors. Firstly it is very easy to add water under pressure to the inlet side of the turbo by using a metered spraybar in the input to the turbo. Water has the low specific heat required to both cool the mixture and impart added power. When water is ingested into a conventional engine the water is turned into superheated steam under combustion and provides and extra „kick“ to the piston. This can be as much as 30% if correctly engineered. In reality 20% is more likely as the water is not always vaporised effectively, but in a turbocharged engine we have the turbine blade in the inlet to give us an added benefit. The water if injected into the inlet side of the turbocharger is firstly atomised by the rapidly spinning blade and takes up the heat from the combination of the compresing of the air and the excess heat from the exhaust impeller side of the turbo. In all the turbo is made for water injection and 20 to 40% extra power can be achieved by setting the water flow at the correct level. A plus factor is that the engine runs smoother and higher ignition advance can be used to good advantage to develop even more power.

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Dangers involved with water/turbocharging.

All the above seems too good to be true and as with all physical items there are a few vital points to bear in mind. If the water feed is too vigorous the danger exists that too large a water droplet will strike the rapidly spinning turbine blade of the inlet impeller and cause severe damage. The inlet impeller is going at up to 200,000 rpm in the Uno Turbo. Even a minute drop of water is like a piece of concrete and if a piece of impeller blade is sheared off the resulting imbalance will immediately destroy the turbo. Therefore it is crucial to increase the water feed very slowly and carefully to get the optimum setting.

14/9/98

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