The best way to understand the rationale behind this new engine design is to start with the basic operating parameters and follow the flow of energy that is transferred to and from the piston. The goal, obviously, is to create the most fuel efficient engine possible. So let’s take a look at each of the strokes or cycles. 

The intake cycle is a parasitic event, meaning that energy from the spinning mass, the cam-stack, is transferred back to the piston in a downward direction. Since the pressure above the piston is either at a slight vacuum or atmospheric pressure, the energy required to accomplish this event is relatively small. (Minimal Negative Energy)

The compression cycle is a parasitic event as well, meaning that energy from the cam-stack is required to accomplish this event. The pressure above the piston in the later stage of the cycle is very high and requires a lot of energy. Because of the large amount of energy required, this event is a major draw on the system.  (Major Negative Energy)

The combustion event in the Frugal engine design occurs at constant volume, which is the most efficient way to combust the air/fuel mixture in an internal combustion engine. By constant volume combustion, we mean that the piston reaches top dead center and then is allowed to dwell at that position until the air/fuel mixture is completely burned. The piston dwell is accomplished by having a constant radius section on both of the cams. This allows the piston to dwell while the cam-stack continues to rotate. A typical dwell period is about 30 rotational degrees of the cam-stack. By combusting the air/fuel at constant volume, we realize a higher combustion chamber pressure along with lower unburnt fuel residues. (Major Positive Energy)

The power stroke in the Frugal engine begins at TDC of the piston. (This is in sharp contrast to the current spark or compression engine cycles which start before TDC.) The pressure above the piston is extremely high in the early part of the cycle. The efficiency of this cycle is determined by the pressure angle that exists between the rocking follower and the drive cam. This pressure angle is determined by the engine designer and produces the offset distance "d" as shown in Fig.2 below. (Major Positive Energy)

The exhaust stroke occurs on the up-stroke of the piston and is a parasitic event. It removes the residual products of combustion from the engine. The pressure above the piston is low during the entire cycle so this cycle requires a minimal amount of energy from the cam-stack.  (Minimal Negative Energy)




As we see from the above descriptions of the 5 Frugal engine cycles, there are 2 up-stroke events, 2 down-stroke events and one event where the piston is at rest.

Our goal is to scrutinize each of the events, concentrating on those which increase or decrease energy in the system in a major way. These events are:  THE POWER STROKE, THE COMPRESSION STROKE, and the COMBUSTION EVENT.  This does not mean that the other strokes are not important; it simply means that our largest gains can be realized if we focus on these three major events.



If we turn our attention to Figure 2 again, we see the offset "d", which is determined by the pressure angle that exists between the roller follower and the cam. This value of "d" is critical in determining output torque generated in this cycle. (Remember that Torque generated is the product of Force multiplied by Distance. The "distance" in this equation is this value of "d".) So, the key to maximizing the output torque is to generate an increasingly large pressure angle which produces a large value of "d" as the piston leaves TDC. This large value of "d" coupled with an increased combustion chamber pressure yields significant power output. (Positive Energy) This increased output is accomplished without consuming any more fuel, producing a more efficient power stroke.


The compression stroke is a purely parasitic event. It begins just after bottom dead center of the piston and ends at TDC of the piston. Our goal is to reduce the amount of energy that flows from the spinning cam-stack back into the piston during this cycle. Any improvements in this cycle become a net energy producer since we will be taking away unnecessary losses. This net energy can then be used to do things such as propel a vehicle, drive a generator or pump, etc. In order to improve this cycle, we must first look at the part of the cycle which consumes the most energy. What we find is that the early part of the cycle, from BDC to about the mid-point of the cycle, consumes very little energy. This is evident if we look at the pressure above the piston during this phase. Data that I have available to me shows that at 70 degrees before TDC of the piston, the pressure above the piston is only at about 1 atmosphere positive pressure (15 psi relative). From 70 degrees BTDC to TDC of the piston, the pressure increases quickly. So our largest gains in efficiency will occur on the compression cycle during these last 70 degrees of cam-stack rotation.  The energy that was created during the power stroke now flows from the cam-stack back into the piston to complete this event. Because the energy flows in the opposite direction during this event, gains in efficiency are realized by minimizing the "d" value during the late stage of this cycle. The shape of the cam that controls the piston up-stroke determines the efficiency of this cycle.  This cam shape is determined by the engineers who design the engine. The results of my design work show the value of "d" can be reduced in this critical period by a little over 50%, the net result being an increase of efficiency associated with this cycle of the same magnitude.


This event occurs due to the ability of this novel engine design to pause or dwell the piston at TDC for a period long enough for the complete combustion process to take place. The application of this technology is significant. Current-day engine designs which utilize the spark and compression cycles will benefit from this technology along with engine cycles that have only existed in the lab. Since the late 1950's, engineers and scientists have been trying to run a gasoline fueled engine on the diesel cycle. This effort is commonly known as HCCI (homogeneous charge compression ignition) and involves injecting a lean mixture of fuel (gasoline) and then combusting the air/fuel mixture via compression ignition instead of spark ignition. This effort has been met with many challenges due to combustion timing variables within the engine. Several of these prototype engines have been displayed in the last few years but none have been offered in production vehicles. The problems occur because the air/fuel combustion takes place before TDC, the timing of which is difficult to control. With the Frugal engine design, the combustion process can be predictably delayed until TDC of the piston. The timing of the event is completely controlled by piston position instead of air or air/fuel temperature. The same process can be applied to diesel engine cycle technology. The net result of burning the air/fuel mixture at constant volume is controlled burn rate which produces significantly higher combustion chamber pressure and lower exhaust pollutants, both of which are highly desirable for maximum engine efficiency.

The two remaining cycles, namely the Intake Cycle and the Exhaust Cycle, may benefit from the above piston motion. It appears that the intake cycle will be enhanced by having the same downstroke piston profile as the power stroke. The exhaust stroke may benefit from the modified upstroke cycle of the compression stroke.  I cannot see how either of these two cycles will be adversely affected by improvements made elsewhere.


Jeff Bonner, BSME
Naples, Florida
May 4, 2015

Animation by Bill Todd


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