On Sat, 14 Feb 2004 12:55:06 -0000, "Roland and Celia Craven" wrote:
Was it low compression ratio that caused semi diesels to need the preheat from cold?
AJH In answer to your question from my limited knowledge which is more based on experience than theory (again limited but I feel it is worth typing). Heavy oils like Diesel, Parrafin, Heating Oil etc.. are relatively reluctant to ignite. Pur some diesel on the floor throw a match in it and the match will go out as in water. Heat the diesel up and it is an all together different matter. Oil engines can be split into roughly three catgories of low, medium and high compression. High compression are what most people call full diesel relying solely on the heat generated by compression to ignite the fuel. Dr Diesel's principle is simple but to do this you need a compression ratio of about 18:1 and a fuel pump and injector capable of overcoming the pressure in the cylinder to atomise the fuel finely enough so that combustion will take place. I will leave diesels alone now as I don't know enough to go into any further detail.
At the other end of the scale is the low compression ignition engine. These engines running at a ratio of around 8:1 require some help to ignite the fuel as they do not produce enough heat from compression alone to cause ignition. As a result some assistance is required. This comes in the form of an external heat source. On the early engines this was a flame applied to the combustion chamber which, as Roland pointed out is isolated in terms of cooling from the rest of the engine, so that when injection occurs the fuel hits the hot surface causing it to vaporise. The resulting gas is far more combustable than the liquid fuel and thus ignites under the low compression. Some of these engines required the lamp to be left on continuously others managed to utilise the heat of combustion to maintain the necessary heat once the engine was running. You also cannot refer to them as having an injector as it was at best a "sprayer". The fuel being pumped in at the correct point in the cycle and sprayed around the chamber or hot-bulb onto the hot surface to maximise vaporisation. Good examples of this type are the Ackroyd-Hornsby engine which employed a hand operated blower to create a hot enough flame as they pre-dated mass produced blowlamps or the "O" type gardner oil engine which was continuous lamp.
As time moved on advances were made in pump/sprayers, shape and construction of the combustion chamber, lubrication etc.. This allowed for increases in compression giving greater efficiency with smaller engines producing higher hp output. This of course brought with it a new set of problems. Surface ignition engines as already mentioned rely on the heat of combustion to facilitate ignition at the next cycle. However when under load the heat generated increases and can lead to pre-ignition of the charge upon injection. This reduces the efficiency and ultimately power of the engine. Also on a practical level an increase in carbon in the chamber results and thus begins a downward spiral. A number of ways were developed to combat this. One of the first and most common was water drip. On some engines this was simply a container of water with a tap and a pipe and the drip rate was adjusted manually to keep the temperature down according to the load. More sophisticated examples employed seperate water pumps linked to the governor that introduced water into the cylinder or combustion chamber directly or via the transfer ports. I have seen a French engine with a nice little brass carb on the side for this job. This was not entirely satisfactory though as water generally doesn't sit well in the engine environment. Other solutions employed were variable timing and spray patterns so that the size of droplet was bigger thus acting as a coolant itself and taking slightly longer to vaporise. Petters in the S-type relied heavily on the design of the combustion chamber and the directing of the scavenge air into the combustion chamber through the use of a very pronounced cut away in the crown of the piston. To their credit it was succesful and of course didn't require manual attention as did many contempories. At this stage the engines were being built with a compression ration of between 10 and 12:1. They carried many advantages with them. They were able to operate on relatively low grade fuels and because they didn't operate at high tollerances didn't require attention from a skilled engineer and proved very reliable. This rendered them ideal for use in locations where the less tollerant more sophidsticated diesel could not realistically be employed.
I haven't touched upon the medium compression engine. These represent the cross over between the two types with compression ratios climbing towards that of the diesel and employing more sophisitcated injection equipment. They all still require some assistance in the form of heat for starting but this is often reduced to a small hollow tube in the combustion chamber with a very small area inside the combustion chamber constituting the vaporiser.
I hope this is of some interest but may hold many inaccurate statements. If you as a reader have spotted one please correct me and then both I as the author and the other readers will not then pass these on inaccuracies to others.
Oh and Roland you are correct about the use of a Bolinder by Petter's. I understand one was purchased and formed the basis for the early V-type engines. Bolinder at this time employing watrer drip. Abandoning this at about the same time as the S-type was introduced in favour of variable spray injectors, higher scavenge pressures and dampers in the transfer port to keep the engine warm when off load.
Regards,
John