THE NEW WAY
Watching the axial engine animation, you will immediately notice four
fundamental differences to the standard engine:
1 - It’s a really cool looking engine where form follows function.
2 - The pistons are equally spaced around the output shaft and parallel to it.
3 - There is no crankshaft, but two counter-rotating barrel cams, one inside the other.
4 - There are no poppet valves, but a couple of rotating devices that function as valves.
Closer inspection reveals that:
5 - The piston stroke lengths are not equal, in that two are longer than the other two, but all the pistons participate in both lengths.
6 - The pistons seem to pause in their motion at both TDC and BDC.
7 - The whole piston and cam assembly can move up or down while the rest of the engine remains stationary, and
8 - The piston and connecting rod’s motion is completely linear and reciprocal.
While only an axial engine design easily allows for all this flexibility, just what does it mean? After all, cam driven axial engines have been studied for close to 100 years, and if they’re better, then where are they? Good question and the short answer is that the previous engines were actually worse. Almost all of the previous cam driven axial engines used a single cam to control piston motion and turn the output shaft. However, under load, where the cam resists turning, the piston and connecting rod would twist (cock) under the pressure in the combustion chamber. This would cause the piston to scrape the cylinder wall and seize, or over time, destroy the engine.
The solution to this problem is to have two cams, one nested inside the other (inner and outer cams) and rotating in opposite directions. This allows the force produced by the burnt AFM to be divided equally between the two cams so no twisting or cocking of the piston takes place. More than that, the piston’s motion is completely linear so that the side loads that would be imposed on it by the geometry of a crankshaft are almost eliminated, which reduces friction, wear, and vibration. The shear forces applied to the connecting rod by the counter-rotating cams is absorbed by the engine block by the “wings” mounted on the lower portion of the connecting rod.
So far so good, but what about fuel efficiency?
Well, before we talk about that we need to talk about variable compression ratios (VCR) and rotary valves. As we learned before, compression ratios determine the effectiveness of the engine. In a standard engine this ratio is fixed and cannot be changed, and is usually less than optimum because of the issue of knock. In the axial engine, it’s an easy and natural evolution to provide this feature (what some have called the “holy grail” of engine design) because all the moving components operate in parallel, and not perpendicular to each other. So by using a linear actuator, both cams, the cam follower, connecting rod and piston sub-assembly can be moved back and forth along the splined output shaft while the rest of the machinery remains stationary. By moving this sub-assembly, the piston crown (top of the piston) moves either closer (higher CR) to, or further away (lower CR) from the header which caps the cylinder sleeve. Thus making the trapped volume of the combustion chamber either smaller or larger. In this way we control the compression ratio through an infinite range defined by the lowest and highest allowable CR, which is designed in. This feature is crucial because the engine’s needs vary so much by type of fuel, load, speed and even atmospheric pressure and humidity.
The animation (variable compression ratio) shows the cam et al subassembly and the splined output shaft. As the subassembly moves upward or downward along the output shaft via the servo actuators, the cylinder sleeves, output shaft and head remain stationary relative to the engine block. In this manner the compression ratio is easily adjusted to suit the operating environment.
In the case of the rotary valve, we decided to use it because they are simply better than the poppets used in current engines. The function of the valve is to allow the AFM to enter the combustion chamber, seal it in and then allow the exhaust to escape. How well they do this has a significant impact on the engine’s performance and is known as volumetric efficiency. Simply stated, the bigger they are, with the least flow impediments, the better they are. Furthermore, they open and close much faster than poppets and they generate a great deal of turbulence in the combustion chamber which promotes a much faster burn rate. The major problem with them, however, is thermal or motion distortion. For a valve to be effective, its tolerances must be very close or tight, but the distortion of the valve through exhaust heat or rotary motion could cause them to seize, which is not a good thing for an engine. Luckily for us, that problem has been solved by what is referred to as the Bishop rotary valve which was first developed for Formula 1 racing. In tests the valve was so successful, that its inclusion generated 10% more power in the engine. Which, by the way, we have not accounted for in the graphs at the top of this article. It was so successful that the F1 governing body (FIA) promptly banned its use. We’re using a modified version not only for its superior performance but also because it’s such a good fit for an axial engine. You may wonder then, why these valves aren’t in current use? Well, the answer is complicated, but basically it comes down to cost. Rotary valves just cost more. But with our design we only need two rotary valves for the four cylinder assembly, where as a normal 4-cylinder engine requires 16 valves and two camshafts.
Note the simplicity of this design. The exhaust port (larger opening) follows the intake port (smaller opening) at a 270 degree angle, and each valve services two cylinders. The valves are driven by a single drive gear and axle, as shown on the animation which eliminates a whole host of moving parts in a regular engine. The sealing is accomplished by a series of biased spring seals that surround the window into the combustion chamber and press against the rotary valve body. They are similar to the seals used in the Wankel engine’s rotor.
Click here to learn more about the Bishop rotary valve.
The following diagram (exaggerated for clarity) reflects the details of the outer cam track in two dimensions. The inner cam is similar but it has two converging/diverging cam tracks which allows the over expansion feature to operate. This is shown in the animation under the heading of over expansion. The cam track taken by the cam follower is selected and controlled by the outer cam track. The cam track follower, in red, is very slightly smaller in diameter than the cam track width, and is depicted close to Point 1, and moves from left to right in this scenario, although in reality, the cam follower is stationary (horizontally) and the cam track rotates from right to left. The cam follower bogie consists of 4 individual cam followers; two on either side of the connecting rod. The cam tracks also consist of 4 individual channels. There are two on the outer cam and two on the inner cam. Each of the outer and inner cams tracks are separated by a very small vertical distance (not shown). In this manner, one cam follower traces the outline of the roof of one channel and the other traces the outline of the floor of the second channel. As a result, each of the individual cam followers rotates in only one direction, and there is no jumping from the roof to the floor of a channel and vice versa. The numbers at the top of the drawing are the degrees of rotation. The blue (floor) line shows the actual side of the track that the main cam (grey/compression, expansion and exhaust) follows, while the black (roof) line shows the track followed by the secondary follower (red/intake). This, hopefully, should make it easier to follow in the narrative describing how the cam and other processes work. Note that the numbers at all the TDC and BDC points are related as follows:
1. TDC, the start of the intake stroke.
1-2. Intake stroke, short leg.
2. BDC, end of intake stroke and beginning of the compression stroke.
2-3. Compression stroke, short leg.
3. TDC, end of the compression stroke and beginning of the expansion stroke.
3-4. Expansion stroke, long leg.
4. BDC, end of the expansion stroke and beginning of the exhaust stroke.
4-1. Exhaust stroke, long leg.
In reality, the number 1 on the left of the diagram and the number 1 on the right are the same points, since the cam is a circular sleeve and contiguous in nature.
Valve timing is included on the diagram where
IVO – intake valve opens
IVC – intake valve closes
EVO – exhaust valve opens
EVC – exhaust valve closes.
Water injection timing is included on the diagram where
WI1 – compression stroke water injection
WI2 – CVC water injection
WI3 – expansion stroke water injection.
Note that the valve timing is mechanically controlled, while the spark ignition, fuel injection and duration, water injection and duration and the variable compression ratio is handled by the engine’s control unit (ecu/computer).
Keeping all this in mind, let’s follow a piston through its paces just
as we did for a regular engine by starting with the intake stroke. Remember
that while the crankshaft determines the piston’s motion and location, here the
cam tracks and the cam follower perform the same function. So, at TDC (Point
1), zero degrees output shaft angle (0 degree OSA, we no longer have a
crankshaft to reference so the term is changed to reference the output shaft
for rotational dimensions), the intake valve opens, and as the cams rotate in
opposite directions, the apparent scissor action of the cam tracks (in this
case the short leg, point 1-2) pulls the piston down to BDC (90 degrees OSA, Point 2) and the enlarged combustion chamber ingests a volume of AFM. Not
much different than a regular engine, except that the output shaft has only
made a quarter (90 degree) turn. This is solely due to the geometry of the
counter-rotating cams; since each piston completes all four strokes for one
output shaft revolution, the output shaft can only turn 90 degrees per
piston stroke. For an eight cylinder engine, each stroke is handled in 45 degrees OSA. This shortened turn has some benefits that will be described later, but
for now it really doesn’t mean much. As the piston leaves BDC, the intake valve
closes (IVC), and the piston heads for TDC (Point 3) (via the second short leg,
2-3). Note that the intake valve closes just as the cam track turns upward.
This ensures a maximum amount of air flow into the cylinder by using the air’s
own momentum. Again the AFM is compressed and heated just as in a regular
engine, and the same pumping losses are encountered. However, in this case, we
inject a tiny amount of water into the AFM (WI1) before TDC (180 degrees OSA,
Point 3) is reached. This cools the AFM and lowers its pressure, which means
that our pumping losses are also lowered and the AFM can withstand higher
compression ratios, which translates into better fuel economy.
By now, the variable compression ratio has been set to its most optimum given the engine’s load, throttle setting and other parameters. Counter-intuitively, any engine is most efficient at wide open throttle (WOT) and high load and least efficient at low throttle settings or partial load. This is partially due to the fact that the throttle restricts the amount of AFM entering the combustion chamber, its pressure is reduced and the engine works as a "vacuum pump", requiring more energy (fuel) just to keep that low pressure. As a result the compression ratio that the engine is designed for is never really reached because there is less AFM in the combustion chamber to compress. And since compression ratio is everything, you lose efficiency when you are cruising.
During the compression stroke the compressed AFM rises to a very high temperature. If that temperature could be reduced, less work would be necessary to compress the AFM and we could increase the compression ratio. So we inject water within the compressing AFM so it vaporizes and cools the compressed AFM down. This improves efficiency in three ways:
Now the other thing that has not happened is that the spark plug has not fired. As we saw earlier, in a regular engine the plug fires before the piston reaches TDC, resulting in negative work, as the pressure is building against the upward movement of the piston. In the axial engine, the plug does not fire until the piston’s motion is at rest at TDC (180 degrees OSA, Point 3). Note, however, that TDC here is actually a flat spot in the cam tracks. Please refer to the animation section entitled constant volume combustion. So the piston actually pauses or dwells without motion for a very short space of time, but it is enough for the AFM to burn completely before the actual start of the expansion stroke. The AFM burn takes place in a combustion chamber of a fixed size which is known as constant volume combustion (CVC) and this has been shown to enhance fuel efficiency. It also ensures maximum applied torque to the output shaft and eliminates all negative work. The final event that occurs in this cycle is a second water injection (WI2) which cools the temperature of the AFM and thus the pressure in the combustion chamber. Which, of course, is also counter intuitive, since this obviously lowers performance. Well you’re right, but it’s necessary to cool the burn temperature to minimize the formation of nitrous oxides, a serious pollutant. Another process is used in regular engines which uses recycled exhaust gases (EGR) and performs the same function, although with less efficiency.
OK, we’re finally into the expansion (power) stroke, which is the first long leg of the cam track (Point 3-4). About halfway down, the third and final water injection (WI3) takes place and it coats the piston crown, the cylinder sleeves, the head and the rotary valve with a very thin film of water. But this water doesn’t flash into steam because the pressure in the combustion chamber is higher than the vapor pressure of the water at its current temperature. What this means is that the water has time to absorb the heat in these parts of the engine, cooling them, eliminates any hot spots and heats the water. Later in the stroke, as the combustion chamber’s pressure falls to below the vapor pressure of the water at its new higher temperature, the water will boil, but in a controlled manner (not all at once) giving a healthy kick to the expansion stroke. Now, remember that we’re on the long leg of the cam track, and that in a regular engine all the strokes are of the same length. They would equal the length of the intake and compression strokes of the axial engine. So what happens here is that the expansion stroke continues on well past the point where a regular engine’s piston hits BDC and the exhaust valve in that engine opens ending any further power generation. With the lengthened power stroke of the axial engine, all that residual pressure from the AFM plus the additional pressure produced by all the water we’ve injected provides a significant increase in usable power and further improves fuel efficiency. This lengthened power stroke is known as over expansion, and is reflected in the animation under that title.
At BDC (Point 4) of the expansion stroke (270 degrees OSA) the exhaust valve finally opens and the burnt gases, considerably cooler and lacking much pressure, are expelled from the combustion chamber, aided by the piston’s motion, via the second long leg of the cam track (Point 4-1), back to TDC (360 degrees OSA, Point 1) where the cycle repeats.
Again there are numerous details that have been omitted, but in principle, that is it. So what have we learned?
Over and above fuel efficiency, there are some other benefits that apply to this engine.
1. Because of its configuration, it will be smaller and lighter than any standard engine with the same power. It’s expected that this engine will have half the weight and occupy half the space of a regular engine.
2. There are fewer moving parts in this engine compared to a regular crankshaft engine so it will be cheaper to build. Most of the savings here relate to the valve train of a regular engine.
3. Since its more fuel efficient, the amount of pollutants, including carbon dioxide, emitted into the environment is directly proportional to the amount of fuel not consumed.
4. Due to the more linear torque curve, the transmission can be built with fewer gears, making it lighter, cheaper and more compact. Current transmissions can have up to 9 gears.
5. The variable compression ratio would allow the use of a smaller and less powerful starter motor since at its lowest compression ratio, less power would be needed to turn the engine over.
6. The torque of the engine is increased since the combustion chamber pressure is applied to a fixed length displacement arm (the distance from the output shaft centerline to the connecting rod centerline) and at right angles to it. In a crankshaft oriented engine, the displacement arm varies from zero to ½ the stroke length and back to zero, and the combustion chamber pressure is always applied at a less efficient angle. Due to this increase in torque, it is entirely conceivable that this engine could replace diesel engines in the future.
7. While this narrative describes a gasoline burning engine, it should be noted that this type of internal combustion engine can use any of the available fuels, excluding high sulphur diesel, from biofuels to LNG if properly designed for.
8. While the engine can be easily scaled up or down, or have more cylinders, this would cause a conflict with the rotary valves, so it may be more appropriate to gang multiple 4 cylinder engines together for very high power requirements such as in ships or locomotives.
9. As it stands, or as near as we can determine, the engine model comes very close to the fuel efficiency standards, by market segment, that this nation has mandated for 2025. Used in hybrid configurations, the numbers greatly exceed those requirements. And, if adopted by industry, further improvements, over time, should allow us to easily reach those objectives.
There is, of course, a price to pay for all of this. Three challenges have been identified and are considered here. The first refers to the piston acceleration curve which is steeper than that of a regular engine. This is due to the fact that they accelerate and decelerate from dead stop to full speed only in the curved sections of the cam tracks, and coast in the straight sections. Since this is a lesser distance than provided by a regular engine whose acceleration/deceleration length is equal to the stroke length, the speed necessarily is higher. However, by properly designing the cam track and keeping rpms relatively low so that piston speeds do not exceed 3 to 4,000 feet/minute, this issue can be successfully mitigated. The second issue is that of the operating temperature of the engine. Standard engines operate at about 100 degrees C, but that is insufficient a temperature to take advantage of the water injection system since the water’s vapor pressure is very low at that heat level. The axial engine would operate from 250 to 300 degrees C in order to ensure an adequate vapor pressure level. While these temperatures are not of any consequence to the engine, they are, in fact, improving the engine's efficiency, they do need to be noted. Any excess heat generated in the engine would be handled by an oil cooler. The third issue is that the water injected into the engine is lost via the exhaust so it has to be replenished on a regular basis. While a closed loop is possible, it adds a degree of complexity to the engine which we felt is not currently warranted. Instead, a micro pore condensation process (US Patent 8,511,072) can be added to the exhaust system which can recover up to 66% of the water expelled. While this doesn’t quite recover all the water that is used in the water injection cycles, it does mitigate it to the point that onboard storage is relatively small.