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HOME/Techtalk #1 - Burning Fuel in Air

4 November 2016 by Harné Heuvelman

We’re all concerned with the power output of our great classic Ducati. The more the better, right? But we do like it smooth and torquey as well… So to get (more) power, where do we start? Where does power come from?

In the end it’s all about burning fuel, petrol to be more exact. The internal combustion engine burns fuel in air and converts the heat produced in mechanical work. Heat creates pressure acting on the surface of the piston which creates the force that turns the crankshaft. What we’re after is the engine that makes the most efficient use (that is the highest force that turns the crankshaft or ‘work’) per unit of fuel. But that is easier said than done…

Let’s start with the fuel itself. Without going into a full chemistry lesson, what happens in burning petrol (or any material) is the breaking up of the chemical compound and creating heat in the process. What is of most importance here when it comes to harness as much heat as possible is 1) the calorific value of the fuel and 2) the mixture between air and fuel.

Calorific value is the amount of energy produced by a complete combustion. Another measure of importance here is the so called stoichiometric value of a substance which is the ratio of air and fuel needed for complete combustion with only H2O, CO2 and heat as a product. For reasons of chemical stability (nitro-glycerine has a very high calorific value but I wouldn’t like to fill my fuel tank with it) and practical day-to-day use and availability (hydrogen has very good burning properties but is hard to produce and carry around), petrol became the fuel of choice.

Petrol has a calorific value of around 45,3mJ/kg and a stoichiometric value of 14.7 meaning that if it is completely burned, a total of 14,7 kgs of air per kg of petrol is needed and that this will result in an output of 45,3mJ of energy. In practice it is found however that an engine gives more power when it is running a little richer than at the pure stoichiometric value so we normally aim for an air to fuel ratio (AF) between 12 and 13. It is seen however that for improving fuel efficiency in modern engines, this ratio is taken well over 18 meaning these engines are running very lean.

In order to harness as much of the energy created by burning petrol, we aim to make an engine that is thermally as much efficient as possible. This thermal efficiency (a ratio between the actual power produced and the potential power available) is closely linked to an engine compression ratio as we’ll see in the diagram in picture 2 (above).

So in order to increase the thermal efficiency, we just need to increase the compression ratio… however, this is limited by detonation or ‘knocking’ which is the combustion of the AF mixture at an unwanted timing. When increasing the pressure inside the combustion chamber, there comes a point that the air/fuel mixture will ignite without a spark (a feature used in diesel engines). This burning will merely result in heat that will not be harnessed into mechanical work. Ignition timing plays a role here as well. Normally we aim for the moment of ignition to happen as the piston moves through Top Dead Centre (TDC) so the spark must be advanced to let this happen. If it is advanced too far this will increase the pressure on top of the compression from the piston.

Different fuels, even different types of petrol, have different properties when it comes to detonation which is given by its octane rating (this is not the octane content as it is often referred to). The higher the octane rating, the more compression the fuel can withstand before detonating. This is why it is recommended to run petrol with a high octane level (100 or more) in racing engines with high compression.

So now that we have determined the desired fuel type, the compression ratio, ignition timing and the desired AF mixture, how do we make that flammable gas and how do we get it inside the combustion chamber?

Enter Bernoulli's principle that states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's potential energy. Without going into the physics behind it, this principle is used in fuel systems by creating a narrowing in the intake (called a venturi) lowering the pressure inside this venturi and so increasing the gas speed. (see picture 3 above)

This increase in gas speed is needed as we have seen before that with a desired AF mixture of 12 to 13 we need a lot of air. How much? Let’s do a small calculation for a Ducati 750 Sport with a total displacement of 748cc or 374cc per cylinder. At 6.000 rpm (100 per second) this means the engine has 50 intake strokes per second so displacing a total of 50x374 = 18.700cc per second. With an intake port diameter of 30mm this means the gas travels through a 7,07cm2 area with a speed of 18.700/7,07 = 2.645,73 cm/sec. or approx. 158 km/h.

For a Ducati 900 Super Sport with a standard displacement of 860cc or 430cc per cylinder, this results in a 21.500cc/sec @ 6.000 rpm and with a standard inlet port diameter of 38mm (11,34cm2 area) gas travels at 1.895,77 cm/sec.

But this is only true with the inlet valve open. In practice the gas speed will rise as the intake valve opens, continue accelerating until the piston reaches top speed and then will start do decelerate coming to 0 as the intake valve closes.

This cycle makes for the pressure waves inside the intake tract and if valve timing is set at an optimum, this feature can be used to fill the engine with as much AF mixture as possible and at the highest possible engine speed. As you see in the calculation above, an increase in either inlet port size or engine displacement slows done the gas flow so in order to make use of a larger capacity an increase in engine speed is needed as well.

In short, engine tuning is often limited by the gas flow and more specific the amount of air. Precisely the reason why turbo’s and super chargers are so popular with many engine tuners.

Back to the carburettor that does the job in making the AF mixture we’re looking for. A carburettor is a smart piece of machinery, beautiful in design like a Swiss watch. It doesn’t just make an air/fuel mixture, it also provides for a difference in this mixture depending on what we ask of it at a certain time. We can divide all the workings of a carburettor and more specific the Dell’Orto PHM and PHF type carburettors into different stages (see diagram in picture 4 above)

A – Idling
The venturi is largely closed by the throttle valve. Petrol is supplied by the idle jet. Adjustment is done with throttle and mixture screw. More about this later.

B – Progression
The throttle valve opens further and the pressure inside the venturi drops making for petrol to rise up through the main jet. This is however not enough in this stage so extra petrol is sucked through a small passage under the throttle valve.

C – Middle
In this stage, the main jet provides enough petrol. The shape of the throttle valve makes for the amount of air inside the venturi and the size of the main jet and needle make for the amount of petrol that is mixed.

D – Full throttle
The needle size plays a role until full throttle is reached. At this point it is only the size of the main jet that makes for the AF mixture.

Extra care should be taken in setting the correct floater needle and floater level as this will make sure the right amount of fuel is present in the float bowl at all times. The Dell’Orto PHM and PHF carburettors are fitted with an acceleration pump. This provides for extra petrol at the moment the throttle is opened suddenly to compensate for the rise in in pressure (and loss in gas speed) when that occurs.

The setting of a carburettor is much depending on how the engine is designed and on the actual use. As we have seen, engine tuning is all about finding an optimum for any circumstance and the setting of the carburettors is very important for the end result. It can’t be said what the exact setting should be for any specific model although you can take the standard fitment as a basis. Further adjustments must be done as a matter of trial and error. Of course the best way to adjust is to go to a Dyna test centre so the effect of any change in setting can be seen directly.

From experience we see however that many Ducati bevel owners struggle with idle adjustment. We always follow this procedure for idle adjustment:
1. Make sure there is enough play in the throttle cables. A minimum of 1mm is required.
2. Warm up the engine by driving a minimum of 5 minutes.
3. Fit a set of vacuum gauges to the inlet manifolds.
4. First set the throttle stop screws by synchronising the vacuum gauges; the throttle screw of the carburettor with the higher vacuum reading should be turned in until the gauges read the same. Alternatively, the throttle screw of the carburettor that has the lower reading must be turned out. Aim for a suitable idle speed and synchronized vacuum gauges
5. Second we set the mixture screw of the rear carburettor by turning it in until the engine speed starts to fall. Now we turn the screw out until we find the point where the engine speed is the highest.
6. The throttle screw setting must now be adjusted again by repeating step 4.
7. Now we go on to set the mixture screw of the front carburettor by repeating step 5 (this time of course only on the front carburettor)
8. With the throttle and mixture set, we go on to adjust the throttle cables. The aim here is to ensure both carburettors are operated at exactly the same time. Open the throttle a small amount while closely observing the vacuum gauges. The cable adjuster of the carburettor with the gauge that moves second must be turned out. Alternatively, the cable adjuster of the carburettor with the gauge that moves first must be turned in.

Of course much more can be said about engine tuning and adjustment. As we have seen it involves quite some time and effort and if when talking about increasing power output dramatically, a lot of knowledge to ensure you make optimum use of the changes. In all we hope this article provides you with some background information. We hope to be there for you if you have any further questions or remarks.

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