Semi-Diesel In answer to A Heggie (see M-Way Swap)

On Sat, 14 Feb 2004 12:55:06 -0000, "Roland and Celia Craven" wrote:

The Semi Tangye diesel,

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.



Reply to
John Macdonald Smith
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Thanks for the input John, I will add a bit that I know.

This is because the match has not enough energy to heat the oil up to its spontaneous ignition temperature or its flash point.

There are two different properties to do with ignition of fuel in air which is why spark ignition engines function differently from compression ignition engines. Petrol actually has a high spontaneous ignition temperature and a low flash point, hence it needs a spark or hot spot above its spontaneous ignition point to start propagating a flame front. Diesel has a high flash point but a low (~400C) spontaneous ignition point, so once the fuel reaches this point it will burn.

I have inadvertently tested these properties, I have spilled petrol on a hot exhaust and been surprised it did not explode and I have had an oil line burst and spray an exhaust with immediate combustion.

I assume the fuel grades in between petrol and diesel (kerosenes and vaporising oils) exhibit properties in between. So whilst Roland's explanation of the semi diesel fits well with reducing the "robustness" and cost of a full diesel and high pressure injection, I did wonder if it were fuel related, after all we are now spoiled with highly specified and refined fuel.

OK so these are semi diesel as the crux is that the fuel is injected into the charge which has separately been drawn into the cylinder. There is no pre mixing but additional heat is needed to reach spontaneous ignition point.

Now this begins to sound more like a pre mixed charge "detonating" instead of developing a flame front from an ignition source. I take it from the later comments you made that some of this was from unburnt fuel still in the cylinder from the last cycle.


Reply to
Andrew Heggie

The 1920s fuels like Russolene and Royal daylight are not significantly different to today's diesel (aka 35 second Gas-Oil). In terms of lubricity they are in fact worse whilst the viscosity and BThU/lb are of the same order. Those early fuels were straight distillates whereas modern fuels are the product of distillation and cracking which both increases the total recovery and reduces the proportion of now more or less unsaleable heavy-oils. I made a mistake in my earlier post. The Tangye M sprays at 30 deg BBDC and is therefore in effect pre-mixed. I suspect this is because in the absence of modern jerk-pumps the near TDC pressure could not be overcome (or perhaps not at a saleable price). The Ogle would seem to be similar. Combustion chamber design was in its infancy and lagged behind petrol engines. Mixing was therefore imperfect. Rudimentary control over both the timing and initiation-point of combustion was achieved by the design and length of the hot-tube. Also since the, effectively, pre-mix could be subject to partial cracking in the cylinder combustion could readily degenerate into knock. This was especially true of the early Ackroyd style engines but generally much less so in two-stroke semi-diesels. Four stroke semis were also made and offer more scope for control over timing although maintaining hot-bulb temperature is more of an issue. Brons style engines make use of the fuel's tendency to fractionate under/heat and pressure and there is an excellent Dutch site covering these weird beasts of which the Rancid Buzzard is one of the few UK examples.

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always just my opinions.... Roland

"Andrew Heggie" wrote in message snip> So whilst Roland's

snip> OK so these are semi diesel as the crux is that the fuel is injected

Reply to
Roland and Celia Craven

I am not exactly sure what you mean by developing a "flame front from an ignition source". If you could add a bit more.

As for the later comments the answer is no there is no reference to unburnt fuel in the cycle. The hot-bulb casting or ignition tube requires heating to around just over 800f/426c just over the ignition point of the diesel. So when the diesel hits the surface it vaporises but ignites at the correct point in the cycle. When an engine is running under load the temp goes up to around 1100f/593c. At temps around and above this the fuel can ignite in part spontaneously before TDC as injection is taking place at ten to twenty degrees in advance of TDC. This pre-ignition causes imperfect combustion and a quick glance at any of the manuals from engines of this type will give advice on how to spot the tell tale signs. The most common being a discernable knock or the hot-bulb glowing red hot. Remember the structural integrity of cast iron rapidly goes down after about 1800f so the engine could in theory burst it's combustion chamber. I've seen my Petter S run red hot. You can bring it on intentionally by adjusting the timing and putting the engine on top load. I do have an extract from an engineering paper of around

1914 which explains it all well with the relevant diagrams. I have it packed away but if I find it I could quote from it a bit more accurately.

I hadn't intended to get over technical so as to concentrate a bit more on the practical developments of the engines. I also am exposing my own failings when it comes to the technical side.


Reply to
John Macdonald Smith

In both compression ignition and spark ignition engines it's important a flame moves through the working gas smoothly, heating it up without creating shock waves. If fuel:air reactions take place by being simultaneously initiated throughout the charge you have knocking. In si engines this is controlled by using a high enough octane fuel to resist detonation and keeping the compression ratio low enough that the rapid rise in pressure once ignition takes place is not high enough to detonate the whole charge. In modern diesels the flame initiates around the first droplets of oil from the injector and primary burning is then sustained during the injection period (which means combustion continues for a longer period in a diesel I think).

It's OK Roland has covered this point in his post, where he points out "injection" is taking place well before tdc, so the charge has become premixed before ignition takes place.

Thanks to you both for the insight, now I'll just have to find one running.


Reply to
Andrew Heggie

It looks like the cup is really a pre combustion chamber, as in an indirect diesel.

and very welcome they are.


Reply to
Andrew Heggie

Well, yes and no. The holes in the Brons cup are much smaller than those in an antechamber diesel. It is specifically designed to enable the lighter fractions of the fuel to start to burn first. The resulting pressure rise blows the rest of the fuel out into the combustion chamber proper, where it too will burn. I believe the small holes mean that not enough air is in the spray cup to allow all of the fuel to burn, otherwise the spray cup carbons up very quickly. This was indeed a problem I had when I owned a Ransomes Wizard. Also, on the Wizard, the injection pressure was derived from the crankcase compression, which with good main bearings would only be as much as 3-4 psi. This meant the fuel was admitted to the spay cup at BDC, so it had plenty of time to heat up and start to fractionate as the piston came up the bore and pressure/temperature rose. As you can imagine, the actual ignition point is not well defined, its very fuel dependent. Starting is truly terrifying, the combustion knock sounds as if the head is going to fly off, and broken gudgeon pins weren't unknown.

On an antechamber diesel, the fuel is injected before TDC at a high pressure. Again, as the antechamber only has small holes in it, the fuel will only partially burn and then blow the rest out into the combustion proper, but NO fractionation of the fuel is involved.

An antechamber diesel is a species of indirect injection engine, and was much favoured by continental manufacturers. It only needs a low( for a diesel) injection pressure, and is very nozzle tolerant. Drawbacks include less than optimum economy, and can be hard starters, even with glowplugs. It is rather different to the better known Ricardo Comet type chamber, but any diesel engine textbook of the 1950's will cover the basics pretty well. I'm not even going to mention Hesselman type engines at this stage :-) I think spark ignited diesel fuel is just too confusing.....


Philip T-E

Reply to
Philip THornton-Evison

"> In both compression ignition and spark ignition engines it's important

I have certainly learnt something myself there.

Many thanks Andrew and if you want to see one running send me a mail and I will start mine up for you if you are ever in the area.


Reply to
John Macdonald Smith

The statement above has finally answered a question that has been bugging me for 30 years.

My parents for a year owned a Norwegian fishing boat that was powered by a 30 HP Union semi-diesel. This engine had a variable spray pattern control on top of the fuel injector that was controllable from the wheel house.

When we bought the boat in Norway it was explained that when the throttle was advanced to above half speed then the spray pattern could be increased to the maximum which was a straight jet of diesel. Why this was necessary was never determined as the previous owner did not speak any English and our Norwegian was very limited.

This we obeyed without question and everything ran well, although one had to remember to close the injector back to a spray otherwise the engine would stall.

We were rudely reminded of this when we were going into a port on the West Coast of Denmark and my father forgot to return the injector to a spray on approaching a dock. Well the engine promptly flooded and quit about 50' from the dock just as he was reversing pitch on the propeller.

The crunch that was heard around the dock had everyone running and we as crew were left sprawled all over the deck. Quite embarrassing and somewhat damaging to the straight wooden stem. Fortunately the boat was immensely strong and did not suffer too much damage.

Thanks for clearing this up for me.

Best regards,

Chris Kessell

Reply to

Yes I am getting a better understanding now. It's a neat idea to add a bit of kinetic energy to the spray where mechanical means are limited.

In fact I believe even direct injection diesels have in effect a two stage combustion, first a diffuse flame at the boundary of the fuel and air and then the combustion of the species of products of incomplete combustion from this.

Why not? Go on.



Reply to
Andrew Heggie

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