4-stroke diesel

Hi,

It's been a while sins I have posted here, occupied with a 4-stroke engine development that was launched for rather small gas engines.
After a while I became more and more convinced that the technology we are using to obtain a higher volumetric efficiency could be used in low revving boat 4-stroke diesel engines.

We have called it the DeepBreatHER technology and the principle is based upon a split first intake stroke.
We have a two phase intake and during the first phase, the engine aspires 90% of his displacement in an identical way as any normal 4-stroke. During the second phase, ports near bottom dead point free the compressed air that was build up in the crank-case.
Drawings are showing an theoretical volumetric efficiency of 170% if only the compressed air of one revolution is considered.

I have two questions;

Is this kind of split first stroke technology ever used?
How close will this theoretical VE of 170% be compared to real world 100 rpm to 1000 rpm diesel engines?

Thanks,
Lemans

 

Interesting concept. How do you address the issue of air escaping back into the crankcase when the piston starts going up? In two stroke engines there is usually a deflector, and the intake air pushes the exhaust out. In this case, seems like the air will enter the cylinder, by then go back to the crankcase as the pressure difference reverses.

 

I see a problem in ports which are closing to fast - let's say, 30° before and 30° after bottom dead point , not really in ports that allow more than time needed to get the pressure equalized.
The pressure depends on displacement and crank-case volume in BDP.
In for this purpose extremely narrow designed engines, the crank-case volume can be down to only 15% of the displacement. In that case, power will be very high, but fuel efficiency will be not that great due to pump loses before the point of open ports is reached.
It all depends on what is wanted most. Power or fuel efficiency.

 

What happens on the power stroke?


The cylinder pressure, on the down stroke, just before the piston reaches your secondary intake ports, will be high due to combustion above the piston, , this high pressure, hot, possibly still burning mixture will be forced at high pressure into the crankcase the moment the piston clears/opens your secondary intake ports

 

If the engine is build for the best brake specific fuel consumption, all fuel will be burned well before the bottom ports are open.
In any case, the exhaust valve will open sooner than the bottom ports are reached. So exhaust gas is already escaping and will draw fresh air into the cylinder, also helped by the pressure that is available in the crank-case. The same is happening in every 2-stroke engine.
If this is not sufficient, extra under-pressure created by an exhaust is a way to help preventing hot gas entering the crank-case, also one-way valves are an option.

One-way valves are also needed if we want to recycle the pressure of the 'lost revolution'.

 

As you say, this is a high power density design (per stroke). It would have rather poor fuel efficiency compared to more bulky designs. Combining with slow speed would completely defeat it's purpose in life. Power density is directly proportional to both speed and the mass flow per rpm. Modern engines use high rpm and turbos get the mass flow rate up. They do the opposite of your scheme to promote efficiency and manage emissions - they leave the intake open into the compression stroke and let some of the intake escape back out, which reduces compression ratio but retains a high expansion ratio. Combined with a turbo and EMS, this gets the job done.

I think this was used in the past in some old aircraft 4 stoke radials. There, the prop fixes the RPM lower than what is ideal from an engine standpoint, and a high power per stroke in a light-weight package at altitude in cold air was handy. But even these engines had a pretty poor power to weight ratio compared to modern engines and were replaced with external supercharger/intercooler systems when they became available.

Unless you are operating at redline, if you have pressure available, you want it operating to improve average cycle pressure, not acting against it. Get the presurized intake air in as soon as possible and remove backpressure from the crankcase during the intake stroke. This is very important if emissions are an issue because peak temperatures are effectively capped.

One difference between small and large engines is the difference in the amount of scavenging that can be accomplished via the Kandency effect. Crankcase induction pumping supplements the weak Kandenacy effect in small displacement motors. But you don't need it in larger displacements. One example would be exhaust pulse pressure charging. We can exploit kenetic energy in the exhaust in bigger machines.

As second difference between small and large engines is in the relative size of components. A low or medium speed diesel has a massive bottom end compared to a small engine. Saving space in the cylinder block has a much smaller effect on the overall package and is therefore not as important.

 

Depends on how you like to use the engine. High power output ( high pressure second intake phase)
or more fuel efficient set-up ( low pressure second intake phase). The used second intake phase pressure can be controlled in two way's. Variable crank-case volume or additional throttle body.

I really can't answer that question. I have searched to find blue-prints of these 'bulky-designs' to duplicate such an engine (CAD) to find out the difference.

I'm afraid and don't follow this entirely. ( partially because I'm a dutch speaking world citizen – partially because marine engines are new to me.. ;-) )

 

The old radial you referring to had only one exhaust valve and the air was drawn into the cylinder by a valve in the center of the piston or ports near bottom dead point in the cylinder - not under pressure but because a vacuum was created when the piston moved down.
The power weight ratio was not that bad. 115 HP@ 1300rpm for only 135 kg.
The leading brand in ultra light and sport-plane aviation, Rotax delivers about 100HP for half of that weight but needs to run their engines at 4 times that speed.

 

I do believe that this is something I need to look into.
Maybe naive but if you need to compress air to a 'diesel usable' level, it should take the same amount of power, regardless how this is achieved.
If a supercharger blows 2 bar into the cylinder during the first stroke, the piston needs to compress that 2 bar and this will ask significant more power than compressing 1 bar.

If we can limit the amount of burned gas entering the crank-case it would be OK.

Good point. It's quit possible that half the size is not important if the engine burns more fuel for every kW delivered.
However, I'm have no clue if this is the case. Reducing moving parts most have some influence on friction – lost power needed to accelerate and decelerate bigger and heavier parts..?
I have no valuable answer on this one.

 

Not certain I understand the goal.

A piston moving down will compress the air under , but is not most of the compressed energy regained on the next up strike?

Many cars will cut off cylinders to load the operating cylinders better , and seem to suffer little pumping loss from the dead cylinder.

Better breathing is always nice , but if you could get atomized fuel into the crank case area and pump vaporized fuel in to the cylinder efficiency would rise.

 

Look up the comparisons between "Otto and Atkinson cycle" gas engines, it may clarify a few things.

 

In this case, no. The energy that forces the air into the cylinder during the second phase is lost, just as any other type of blower you may think of. I believe that even a turbo should have a minor effect on 'lost power' for compressing the air. The pressure drop during exhaust stroke is hold up slightly.
On high revving small engines, the turbo is fed by pulses, escaping as soon as the exhaust valve opens. No significant loss in that case. No idea what happens in those very big marine diesels but I don't see the same effects spinning those big turbo chargers. It think it's the constant flow of exhaust gas that spins these chargers.

Never heard of this in diesel applications but for all 'spark-ignition' fuels it can't be a problem.

 

Interesting reading. You have the real thing and besides that, the 'valve' controlled 'Atkinson' cycle as used in Toyota engines. It explains the high compression ratio of the Toyota 1.2 'Atkinson' petrol turbo engine. Personally I think that the Toyota engine cannot be called 'Atkinson' engine.
Besides that, it's a smart way to control the 'virtual' compression ratio.

Thanks Jim

 

I have been reading this thread based on a single piston but when considering 2 or more pistons, I am not sure if the crankcase would in fact pressurize with say 2 pistons.
Consider a 2 piston engine
If one piston is moving downward and pressurizing the crankcase, a second piston will be moving up and creating the same loss of pressure at the same time. Ie the swept volume will be the same.

In a engine, with multiple pistons, the crankcase does not pressurize, unless there is some blow by.

So are you somehow going to isolate each cylinder?

 

The crankcase has to have labyrinth seals like a two stroke.