The SUPAIRTHERMAL Nordberg Engine
Authored by: Norman Nelson, Aug 2005
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In Aug 2005, Mr Norman Nelson of NAK Engineering submitted this paper on the Nordberg Engine to www.dieselduck.net, to offer further details on the Miller Engine. It appears that the paper was from a scanned source, so some of the equations below are a little wonky, but I believe you will get the Idea. - Martin
THE SUPAIRTHERMAL NORDBERG ENGINE
The Nordberg Engine has a unique system of high pressure supercharging. While high pressure supercharging is not limited to Nordberg Engines, the Nordberg engine has exclusive mechanical features that enable it to operate over a large range of loads and speeds with the engine automatically adjusting itself to each load condition to maintain nominal temperatures, pressures and clean combustion of fuel. This system is vital to the performance of the engine and it is necessary that operators of ASUPAIRTHERMAL@ Engines understand the basic principles of the system. Without the understanding it would, at times, be difficult to determine if the engine is adjusted and operating properly.
The devices on the engine are quite simple in themselves and need little explanation. However, to understand the proper functioning of the equipment it is necessary to delve into what they are intended to do. A thorough study of the principles would involve rigorous mathematics and thermodynamics which are far beyond the scope and intent of this paper. Rather, we will attempt the explanation in simple terms and using the least complex mathematics although some is required to emphasize certain points.
Supercharging occurs when the density of the cylinder air charge is increased thereby providing more oxygen to burn a greater amount of fuel. The object is to increase the power capability of the engine beyond that obtainable when the engine is naturally aspirated. The naturally aspirated engine can obtain only as much air in the cylinder as the piston can draw in while the inlet valve is open and the piston moves downward from the top of the stroke to bottom of the stroke. To better understand and appreciate the supercharged and ASUPAIRTHERMAL@ Engine we will begin by taking a closer look at the naturally aspirated engine.
A few degrees before TDC firing the fuel is injected and begins to burn. As the piston passes TDC and moves downward the burning fuel raises the pressure in the cylinder to about 1.4 times the compression pressure and as the piston moves downward the force of this high pressure gas imparts high turning effort to the crankshaft. This is the source of the engine's power. Before BDC is reached, the exhaust valve opens and as the gas rushes out of the cylinder its pressure rapidly falls to near zero gauge pressure. Opening the exhaust valve well before the bottom of the stroke appears, at first, to be wasting energy. However, you will notice that as the piston nears BDC relation between crankshaft and connecting rod is such that the force in the connecting rod applies very little turning effort, or torque, to the crankshaft. Also while we are observing the engine in slow motion here, actually, a very small fraction of a second is all the time available to blow down the pressure in the cylinder. If the exhaust valves were held closed until the bottom of the stroke then relatively high pressure would exist in the cylinder as the piston starts its upward stroke and the force of this pressure would oppose the motion of the piston and nullify any advantage in later opening of the exhaust valve. Further, the valves cannot be opened and closed instantly and while the valve begins to open at one point the crankshaft must turn through several more degrees before the valve has opened sufficiently to permit free flow of gas.
As the piston starts its upward stroke the exhaust valve is wide open and the motion of the piston forces the spent hot exhaust gas out of the cylinder. Near TDC the inlet valve begins to open and the exhaust valve has started to close. For a short period near TDC both valves are open. The piston is moving very slowly near TDC and there is little flow of gas either into or out of the cylinder. Just after passing TDC the exhaust valve is closed and inlet fully open. The downward motion of the piston draws in a fresh air charge through the inlet valve. You will notice that the inlet valve is held open well past BDC. The downward motion of the piston on intake strokes creates a partial vacuum in the cylinder which causes air outside the engine to rush in, in an attempt to bring the air pressure in the cylinder up to atmospheric or zero gauge pressure. Thus, while the piston is slowed to a stop near BDC we leave the inlet valve open so that the pressure in the cylinder will come up as near as possible to atmospheric pressure. Obviously, the higher the air pressure in the cylinder the more air and oxygen available to burn the fuel.
As the piston starts its upward stroke the inlet valve closes and the air is trapped and compressed. Compressing the air increases its temperature to the order of 1000EF. so that when the fuel is injected just before TDC it ignites and burns spontaneously to further raise the pressure for the power stroke which starts as the piston passes TDC.
Now, imagine that our engine is running and let's see if we can find any shortcoming in the naturally aspirated engine and what we can do to increase its power. First, let's assume the engine is running at constant speed and rather a light load and there is plenty of air in the cylinder to burn the amount of fuel required at this load. For the sake of illustration, we will assume that combustion is nearly complete such that the exhaust gas contains a high percentage of completely burned fuel in the form of C02 (Carbon Dioxide). The fuel provides carbon (Symbol C) and the air supplies oxygen (Symbol 02). Thus, combustion would be:
(Cylinder) (Yields) (Exhaust)
C + 02 C02 + CO + 02
Free oxygen in the exhaust is the result of having excess oxygen available and C0 (carbon monoxide) and free carbon in the exhaust combustion is not complete. It is virtually impossible to disperse and mix the fuel and air in a manner to obtain ideal combustion and we will always find some C0 and C in the exhaust but they should be present in small amounts.
If we now increase the load on our engine then to maintain speed we must increase the amount of fuel. With increases in load and fuel we would find the 02 in exhaust decreasing and C02 increasing. The C0 and C content would increase slightly. We can continue to increase load until we note a sharp reduction in exhaust 02 and marked increase in C0 and C. We have now applied about all of the load the engine can carry for while we can continue to increase fuel, the amount of air available is fixed at that amount the pistons can pull in on intake stroke. Continuing to increase load and fuel will so increase the free carbon in the exhaust that the exhaust gas will appear black. At this point we say that the engine has reached its smoke limit or torque limit.
Further increase in fuel causes the combustion process to deteriorate badly and to the extent that the engine=s power will decrease. At some point just below the smoke limit we reached the maximum power capability of our naturally aspirated engine.
Now let's look at some of the reasons why our engine ran out of air for proper combustion. First, we know that the engine can only draw in as much air as the piston displaces on the intake stroke. However, at the bottom of the intake stroke we do not have the cylinder as full of clean cool, dense air as we might suspect. The engine has a volume ratio (compression ratio) of 11:1 which means that the volume of the cylinder at bottom of the stroke is twelve times the volume at top of the stroke. This means that the piston starts the intake stroke there is trapped in the cylinder hot, spent gas from the previous power stroke. When the piston reaches bottom of intake stroke then about 1/11 of the gas (9%) is spent gas from previous combustion. Additionally, this residual exhaust gas is hot and it mixes with the incoming air, raising its temperature and lowering the air charge density which further reduces the amount of oxygen in the air charge. Obviously, if we are to increase the engines power capacity we will have to supply more air for combustion.
The first step is to provide an air pump to force air into the engine during the intake stroke. This could be either an engine driven or electric motor driven pump but either of these requires large amounts of power which must be supplied from the crankshaft and absorbs a great deal of the additional power to be gained by supercharging. Instead, the exhaust gas driven turbo-compressor (turbocharger) is used as it utilizes waste energy in exhaust and takes very little power from the crankshaft in the form of exhaust back pressure. Now, let's suppose that we have applied a turbocharger that will provide, say 3 PSIG inlet air manifold pressure at the load at which the naturally aspirated engine had run short of air as evidenced by smoky exhaust. Let's see if we can determine if this degree of supercharging will enable the engine to produce a significant increase in power.
First, consider the following equation which defines all the conditions of a gas at the amount we are observing it: PV = WRT
Where: P = Absolute pressure (Gauge + Atmospheric)
V = Volume
W = Weight of Gas in V Volume
R = A constant for gas (53.3 for air)
T = Absolute temperature, degrees in Rankine (EF + 460E)
By using proper values for P, V, and T of the air existing in the cylinder at bottom of intake stroke we can calculate the weight of air trapped in the cylinder and from that determine exactly how much oxygen is available for combustion. However, we will use the equation only to illustrate the effect of supercharging. By use of subscripts we will designate the naturally aspirated conditions by subscript 1 and the supercharged engine by subscript 2. The laws of mathematics permits us to write the equation for both engines and then divide one by the other thus:
P2 V2 = W2 R2T2 or P2V2 = W2R2T2
P1 V1 = W1R1T1 P1V1 W1R1T1
V2 = 1 as both volumes of the cylinder when the piston is on BDC are equal,
and, R2 = 1 as the gas in both cases in air.
This simplifies our equation to: P2 = W2 T2 or W2 = W1 P2 T1
P1 W1 T1 P1 T2
For the moment we will consider T1 = 1. In the case of the naturally aspirated engine the 9% T2 exhaust trapped after exhaust stroke raises the temperature of the fresh air charge to above ambient and for the supercharged engine compressing the air to 3 PSIG will also raise its temperature - thus for the sake of illustration we will consider T1 = T2 and our equation has now been reduced to: W2 = W1 P 2
With the inlet valve remaining open for a period past BDC we can assume that the cylinder pressure at BDC will be atmospheric, 14.7 PSIA, for the naturally aspirated engine and 17.7 PSIA (3 PSIG = 14.7) for the supercharged engine, then: W2 = W1 17.7 = W1 x 1.2
This shows that by raising the inlet manifold pressure from zero gauge to 3 PSIG we can place 20% more air in the cylinder which would enable us to place a significantly larger load on the engine. But there is still another benefit to supercharging. Recall that there is a period near TDC after exhaust when both inlet and exhaust valves are open. By properly timing this overlap period in the supercharged engine we can use the positive manifold air pressure to scavenge the cylinder, i.e., force all the spent exhaust gases out of the cylinder and into the exhaust manifold. Also recall that we showed earlier that the trapped exhaust gases constituted 9% of the intake air charge in our naturally aspirated engine. Thus, by supercharging to a manifold pressure of only 3 PSIG we have given the engine nearly 30% additional air and the advantage of even low supercharging are obvious.
By careful design of the exhaust manifold system and sizing of the turbocharger we are able to develop intake manifold pressures of 30 PSIG. The temperature of this air is more than 300EF but we cool it down to a usable temperature by placing a cooler between the turbocharger and engine. However, there is a usable limit to the amount of manifold pressure than an engine with fixed valve timing can use. To show this let=s consider the equation:
Pt = Pb (Vb) 1.38
This equation defined the rise of compression pressure starting at BDC with Pb pressure in the cylinder rising to Pt pressure at TDC. The expression Vb is the volume ratio of the engine Vt which we haven taken as 12. Thus (11) 1.38 is equal to 27.5 and:
Pt = Pb x 27.5
For our naturally aspirated engine Pb = 14.7 PSIA and Pt = 455 PSIA or 440 PSIG. With 3 PSIG supercharging Pt = 17.7 x 27.5 = 485 PSIA or 470 and for the same engine with 30 PSIG manifold, Pt = 44.7 x 27.5 = 1230 PSIA or 1215 PSIG. At this level of compression pressure the firing pressure would be of the order 1700 to 1800 PSI and the engine would be subjected to very great mechanical and thermal stresses although the cylinder would have 2.7 times more air in it than with the naturally aspirated engine.
Clearly, we must either reduce the degree of supercharging or find some way to make the 30 PSI manifold air acceptable to the engine. Let=s now determine an acceptable value for compression and manifold pressure and perhaps a solution will be apparent. On the basis of keeping pressure and temperatures within the cylinder to a level commensurate with good service life we shall limit compression pressure to 1000 PSI. Going back to our equation for compression pressure
Pt = Pb x 27.5
1000 + 14.7 = Pb x 27.5
Pb = 1000 + 14.7 = 36.5 PSIA or about 22 PSI gauge
27.5 at BDC
From this it appears that we cannot use a manifold pressure above 22 PSIG. However, the Nordberg engineers were not satisfied with this limitation and subsequently devised the unique "SUPAIRTHERMAL" system for utilizing high pressure supercharging . To do this they borrowed a page from the steam engine handbook, that is, they applied the principle of steam valve cutoff. In the steam engine high pressure is admitted to the cylinder and at some point in the pistons stroke, dependent on load, the steam valve closes and the steam expands to low pressure at bottom of stroke. Applying this principle to the "SUPAIRTHERMAL" Engine we admit full manifold pressure to the cylinder and then close the inlet valve before BDC so that the air is expanded to 22 PSIG at BDC. By properly controlling inlet valve closing we can maintain 22 PSI air pressure at bottom of stroke for any given manifold pressure above 22 PSI. There are many advantages to this but before discussing that let=s see how the closing of the inlet valve can be automatically controlled while the engine is running.
A most important feature is that while we control the inlet valve closing to expand any given manifold pressure (above 22 PSI) down to 22 PSI we are also changing the timing of the opening of the inlet valve. The inlet cam has a duration of 250 crankshaft degrees. At full retard of linkshaft the inlet closing is 20 ABCD so the inlet valve opened at 50BTDC. The exhaust valve closes at 60E ATDC so the overlap period at full retard is 110 crankshaft degrees. Without positive manifold air pressure from the turbocharger this overlap period is of no advantage and, in fact, the late exhaust valve closing gives rather poor cylinder filling when at very low load.
This does not bother the engine as there is more than ample air for the fuel required to idle. We have already explained that as the manifold air pressure rise, air from the manifold will sweep across the combustion chamber during the overlap period to sweep out the spent exhaust gases. In addition to purging the combustion space the flow of cool air effectively cools the entire combustion space including valves. Now, as the load increases, we burn larger quantities of fuel and this causes the combustion space temperature to rise and at the very high loads we need all the combustion space cooling we can get. When manifold air pressure rises above 10 PSI the linkshaft actuator begins to advance the inlet timing and in so doing it advances inlet opening as well as closing. This increases the overlap duration and with increasing load we have increasing flow of cool air across the combustion space. At 30 PSI manifold the linkshaft would advance ti 40 BDC and the overlap duration would be 170 as compared to 110 at light load.
Another advantage to the SUPAIRTHERMAL Engine is that it gives the most effective method of filling the cylinder with the greatest weight of air. To illustrate this let=s assume that the engine has fixed valve timing and to control maximum compression and firing pressure we are limited to 22 PSI manifold. If the manifold air temperature is, say 120EF then at BDC we have a weight of air in the cylinder commensurate with the condition of 22 PSI and 120E. However, in the SUPAIRTHERMAL Engine we would partially fill the cylinder with air at 30 PSI and 120E but after expansion the air charge conditions would be 22 PSI and about 30EF. Under these ideal conditions the SUPAIRTHERMAL Engine would have about 20% more air by weight due to the air charge having greater density due to its lower temperature.
For the Engine Operator - he may think of the linkshaft system as a device to give constant compression pressure above about75% load. At lower loads the manifold will be below 22 PSIG and with the linkshaft at full retard, compression will vary as manifold pressure varies. That is, at idle compression will be 450 PSI which will then increase as manifold pressure increases until 22 PSI manifold is attained. Above this point the linkshaft will advance with increasing load and manifold pressure such that the compression stroke is started at 22 PSI to give a constant 1000 PSI compression. Failure of the linkshaft to move can cause enormous pressures and temperatures in the cylinder and over advancing will cause lowered compression.
The operator should familiarize himself with the program of operation for the linkshaft system. The instruction manual provides a curve showing the relation between manifold air pressure and inlet valve timing.
At the forward end of the engine is a scale and pointer attached to linkshaft which reads directly in inlet valve closing as related to BDC. This, at any load producing manifold air pressure above about 22 PSI, read the linkshaft pointer position and from gauge board read manifold air pressure, entering the curve with these two valves should plot a point which falls between the two lines of constant compression. For the engine we have discussed here the upper line would represent a constant 1000 PSI compression and the lower line about 975 PSI compression.
While the operator may think of the SUPAIRTHERMAL system simply as a means to provide constant compression pressure, to the engine designer it has the more important function of increasing the engines efficiency allowing it to convert a maximum amount of the fuels chemical energy into useful work at the flywheel. To illustrate how the efficiency is increased with high manifold pressures consider the two indicator diagrams of Figure 1 and Figure 2.(attachments)
Figure 1 illustrates the cycles of the low pressure supercharged engine and Figure 2 that of a high pressure supercharged ASUPAIRTHERMAL@ Engine. The diagrams are plots of cylinder pressure versus volume (piston stroke) and knowing that PV = LB/FT2 x FT3 = LB F = work, then the areas of the diagrams are proportional to the indicated work or indicated horsepower developed in the cylinder. This, in Figure 1 the total energy liberated by burning the fuel and expanding the hot gas is represented by the area A B C D under expansion line. This is termed positive work as energy is being delivered to the flywheel. When the cylinder air charge is being compressed from H to B this is energy taken back from the flywheel and termed negative work. This negative work is represented by the are A B H D under compression line and the net positive work available at this point is the difference between area under expansion and compression lines - shaded area H B C. There is another negative work area A E F D which is the work required to push the exhaust gas out of the cylinder and termed pumping loss. However, this is more than offset by the positive work area A G H D which is derived from the pressure exerted on piston by incoming air from manifold. The difference in these two areas is the positive work area E G H F (shaded) and thus the net positive work of the cylinder is shown as the shaded area.
Now consider the SUPAIRTHERMAL Engine of Figure 2. The same reasoning shows a net work area enclosed by the expansion and compression lines (shaded) as in Figure 1. However, the difference in areas between exhaust pumping loss and cylinder filling lines gives a larger positive work area EG G=HF (shaded) because of higher manifold pressure. This in itself indicates the SUPAIRTHERMAL Engine is developing greater power for same fuel input.
The important point here is that at point G= the inlet valve closes and the air is expanded down to point H and then recompressed to point G=. This shows rather than the flywheel supplying the energy to compress the air from H to G=, that energy came from the turbocharger which in turn extracted the energy from waste heat in the exhaust gas. Thus, the flywheel has only to supply the negative work of compressing from point G= to B. This, in effect, is the basic thermodynamic principle of the SUPAIRTHERMAL Engine, i.e., the engine expands air at high volume ratio (more positive work) and compresses air at a lower volume ratio (less negative work) and in so doing achieves a very high efficiency.
In Figure 1 the engine expands air from volume V1 to a volume V2 such that expansion ratio = V 2 = 12. V1
However, it compresses air from volume V2 to V1 for a compression ratio of V 2 = 12 or equal V1 expansion and compression ratios. The SUPAIRTHERMAL Engine expansion ratio is also V 2 = 12 but the effective compression ratio is V2 and about 8 - expansion ratio is higher V1 V1 than compression ratio.
The effect of these two ratios can be dramatized by visualizing that in some way we could devise a turbocharger that could extract enough energy from exhaust heat to pump the air manifold up to 1000 PSI at about 1000EF. When the inlet valve opened at point E the cylinder would fill with this high pressure air brining the cylinder pressure up to point B. We would immediately close the inlet valve and the cylinder filling stroke would then simply be expanding along compression line B G= H followed by compressing along the same line back to point B at which time fuel would be injected and power stroke begun. In this event, the engine would not have to do any work of compression and the net indicated work of the cylinder would be the entire area E G B C H F.
The expansion ratio would still be 12, but compression ratio would then be V 1 = 1.
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