The Lugo engine -- Fulfills the goals pursued by Atkinson, improves both Otto and Diesel engines, can achieve effective variable compression ratio (VCR) with associated exhaust gas recirculation (EGR) and, most importantly, provides a solution to the problems faced when implementing Homogeneous Charge Compression Ignition (HCCI). The new Lugo engine, with its epicyclical movement provides positive torque between 15º before Top Dead Center (TDC) and TDC, where the torque is equivalent to that of a conventional Otto or Diesel engine 16º after TDC.


In this new engine, all combustion events that take place between 15º before TDC and TDC become positive torque and not engine-damaging activities. This feature, in addition to reducing pre-ignition and knock issues, is a potential key to making HCCI a commercial reality. The Lugo engine also offers the increased efficiency and pumping loss reductions associated with being a true mechanical Atkinson cycle with shorter intake/compression strokes and longer expansion/exhaust strokes.

Increasing the efficiency of the internal combustion engine through re-conceptualizing it has been a goal of automotive engineers and entrepreneurs ever since James Atkinson in 1887 [1] tried to circumvent the Otto patent. What Atkinson proposed to do was to achieve a much more efficient engine by significantly increasing the length of the power stroke compared to that of the compression stroke to maximize the energy output. Yet, in spite of about 1500 patents and several attempts by industry it has not yet been possible to commercially implement a simple mechanism capable of fully producing the Atkinson cycle.

An alternative direction pursued, is variable compression ratio (VCR) which allows the engine to change the compression ratio depending on the performance needs of the vehicle. There have been many contributions to the development of engines with VCR capabilities [2,3,4,5,6,7,8,9,10] of which the 2002 patents by Rabhi [2] (MCE-5) and Gomecsys  [4] are the best known. The advantages of VCR have been summarized by Roland Gravel [11] at the US Department of Energy (DOE) “The VCR engine is optimized for the full range of driving conditions, such as acceleration, speed, and load. At low power levels, the VCR engine operates at high compression to capture fuel efficiency benefits, while at high power levels, it operates at low compression levels to prevent knock”



An alternative direction pursued, is variable compression ratio (VCR) which allows the engine to change the compression ratio depending on the performance needs of the vehicle. There have been many contributions to the development of engines with VCR capabilities [2,3,4,5,6,7,8,9,10] of which the 2002 patents by Rabhi [2] (MCE-5) and Gomecsys  [4] are the best known. The advantages of VCR have been summarized by Roland Gravel [11] at the US Department of Energy (DOE) “The VCR engine is optimized for the full range of driving conditions, such as acceleration, speed, and load. At low power levels, the VCR engine operates at high compression to capture fuel efficiency benefits, while at high power levels, it operates at low compression levels to prevent knock”




Another important concept, introduced by Onishi et al in 1979 [12], is the Homogeneous Charge Compression Ignition (HCCI), which, rather than by spark, produces spontaneous ignition by compressing a homogenous mixture of fuel and air. The promise is to decrease emissions significantly and to bring a gasoline engine to the efficiency level of a diesel one.

Since the 1983 Najt and Foster [13] pioneering paper on HCCI, the automotive industry has done extensive research on the subject. SAE alone has over 2000 papers on HCCI. Yet today the authors are unaware of any commercially successful HCCI engine on the market. In a 2008 interview, Dr. Najt presented a prototype car with a dual HCCI/spark ignition engine and he estimated that commercial HCCI was at least 10 years away.


One of the main reasons for pursuing such an ignition method was recently stated by Xingkai Lu et al. [14]: “HCCI combustion is considered to be the most promising clean combustion method with high efficiency that will be able to meet future emissions regulations.” Epping et al. [15] had long since listed and discussed its advantages in relation to other combustion systems, such as spark ignition (SI) and compression ignition direct injection (CIDI):

“The advantages of HCCI are numerous and depend on the combustion system to which it is compared.

Relative to SI gasoline engines, HCCI engines are more efficient, approaching the efficiency of a CIDI engine.
This improved efficiency results from three sources: the elimination of throttling losses, the use of high compression ratios (similar to a CIDI engine), and a shorter combustion duration (since it is not necessary for a flame to propagate across the cylinder).
HCCI engines also have lower engine-out NOx than SI engines.
Although three-way catalysts are adequate for removing NOx from current-technology SI engine exhaust, low NOx is an important advantage relative to spark-ignition, direct-injection (SIDI) technology, which is being considered for future SI engines.
Relative to CIDI engines, HCCI engines have substantially lower emissions of PM and NOx. (Emissions of PM and NOx are the major impediments to CIDI engines meeting future emissions standards and are the focus of extensive current research.)
Another advantage of HCCI combustion is its fuel-flexibility. HCCI operation has been shown using a wide range of fuels.”

The problem is that, in spite of all those advantages, HCCI has not been easy to implement.



Najt & Foster [13] identify the principal problem faced by researchers into HCCI: “Increased compression ratios ... enable lower delivery and/or lower initial charge gas temperature conditions to be successfully ignited, but they also result in dramatically higher and more violent energy release rates”. As can be seen from Figure 1, the energy release takes place before TDC and all the torque generated as a result of this release is negative and tends to do negative work trying to stop the engine.

Figure 1. From Najt & Foster [13] Ilustrating the problem identified by the authors as the Reciprocating Piston Engine Impediment (RPEI)

The authors of this paper have termed the problem illustrated by Najt & Foster in Figure 1 as the “Reciprocating Piston Engine Impediment” (RPEI).
The RPEI results from the reciprocating piston engine having negative torque before TDC (Figure 2A) and zero torque at TDC (Figure 2B). Thus, in reciprocating piston engines, the optimum combustion events happen at the least advantageous mechanical moments. Consequently, RPEI has compelled HCCI researchers to spend enormous efforts in finding ways to control combustion timing precisely, because uncontrolled combustion before TDC is counter-productive for the engine. Reciprocating piston engines have a narrow window of opportunity before TDC where combustion events will be converted advantageously into useful work.

Figures 2A and 2B. The Reciprocating Piston Engine Impediment (RPEI)


This paper describes a design developed and patented by the authors, jointly with Luis Marino Gonzalez [16,17,18], which we believe can successfully overcome the difficulties faced in implementing HCCI and presents additional advantages.




The Lugo design, proposed and patented by the authors [16,17, 18], incorporates a novel variable stroke mechanism which potentially eliminates RPEI by providing positive torque between 15º before TDC and TDC. The Lugo engine would convert all combustion events that take place in that interval into useful work, hence effectively controlling knock and pre-ignition issues. This suggests that compression ratios up to and in excess of 17.5:1 are possible.


The Lugo engine technology consists of a cam mounted on the crank pin of the crankshaft. This cam rotates about the crank pin via a fixed pinion gear (attached to the engine block) and a crown gear attached to the cam, the crankshaft rotates clockwise and the crown gear rotates counter-clockwise at one half the speed of the crankshaft so that, for every two turns of the crankshaft, the crown gear will turn once. The crankshaft and the cam produce a “resultant crankshaft” that is the vector sum of the crankshaft and the cam.

Figure 3. Comparison of locus of points described by the connecting rod big end of the Lugo and the standard reciprocating piston engine


Figure 3 shows the locus of points described by the connecting rod big end of the Lugo Engine and the reciprocating piston engine. It also shows that the locus of points for the Lugo engine is an epicyclical curve while that of the standard reciprocating piston engine is a simple circle. The crankshaft, cam and “composite crankshaft” are also shown. Note that the “composite crankshaft” is shown as a ‘dotted line’ because it is simply the vector sum of the crankshaft and the cam.


Figure 4. Components of the epicyclical locus of points described by the connecting rod big end of the Lugo engine


Figure 4 illustrates the four components of the locus of points curve of the Lugo engine where the intake and compression curves are significantly shorter than the expansion and exhaust curves. This difference generates the Atkinson cycle feature of this engine that will be discussed later.

It can also be seen that the crankshaft vector and the cam vector sum and subtract to produce a “composite crankshaft” of varying length. The Lugo engine has two distinct TDCs; the exhaust/intake TDC, and the compression/expansion TDC. These two TDCs are each, 20º on opposite sides of the cylinder center line when viewed as a function of the crankshaft position.


Figure 5 shows that with the crankshaft at 35º before the vertical (cylinder center line), the “composite crankshaft” (product of the vector sum of the crankshaft vector and the cam vector) is vertical (aligned with the cylinder center line) producing zero torque, the same as Figure 2B above.


Figure 5. Lugo engine composite crankshaft at 15º before top dead center and the crankshaft at 35 º before cylinder center line.

Figure 6. Lugo engine at top dead center, Crankshaft at 20º before vertical and composite crankshaft at 13º past the vertical


Figure 6 shows the Lugo engine at top dead center. Here the crankshaft is at 20º before the vertical while the “composite crankshaft” is a full 13º ahead of the vertical providing significant positive torque.


Figures 5 and 6 illustrate that any combustion event that occurs between 15º before TDC and TDC will be converted into useful work. This feature would give the engine design engineer a full 15º before TDC to implement HCCI ignition strategies, without the problems associated with the RPEI.


Figure 7, shows the two Bottom Dead Centers (BDCs) of the Lugo Engine which although they are the same when viewed as a function of the crankshaft position, the pistons are physically at different locations for each of the two BDC. This figure illustrates the Atkinson feature of the Lugo engine where the intake/compression strokes are shorter than the expansion/exhaust ones.


A key feature of the Lugo engine is that  the intake/compression strokes are 160º ‘long’ and the expansion/exhaust strokes are 200º ‘long’, when measured from TDC to BDC on the crankshaft.


Figure 7. BDC’s intake and expansion for the Lugo engine




Lugo Assembly


The Lugo engine mechanism requires the addition of a pinion gear and a crown cam to each cylinder of the engine


Figure 8. Figure 8. Pinion gears installed in the engine block


The pinion gears are rigidly attached to the engine block (although they could be rotated for VCR applications), concentric to the crankshaft axis, as shown in Figure 8. In the case of the prototype 4 inline engine shown, the pinion gears were mounted as pairs on opposite sides of bearing blocks 2 and 4.


The crown cams (Figure 9), which consist of a crown gear and a cam off-set from the center axis of the crown gear (18 mm in the case shown in Figure 9), are mounted onto the crankshaft pins so that the crown gears mesh with the pinion gears and the crown cams are able to rotate about the crank pin.

The connecting rod big ends are mounted onto the cam of the crown cams in the same manner as they are to the crank pin on a conventional engine.


Figure 9. Crown cam showing the crown gear and the cam


Figure 10 shows the partial engine assembly with the crown cam for the first cylinder mounted on the crankshaft in the engine block.


The Lugo crankshaft, Figure 11, which was made of high strength steel, has a reduced crank arm (30 mm compared to 43.75 mm for the standard crankshaft), In the Lugo engine prototype shown, at BDC (expansion), the crank arm of 30 mm sums to the cam of 18 mm for a total crank arm of 48 mm. It is significant that the Lugo crankshaft has no counterweights, which results in a weight of 7.3 Kg, versus the 17.4 Kg of the standard production engine crankshaft.


Figure 10. Partial assembly of crankshaft, crown, cam and pinions in  the engine block


Figure 11. The standard production crankshaft (top) and the Lugo high strength steel crankshaft (bottom)


Figure 12. Prototype mechanism completely assembled in the engine block






The Lugo design has the benefit of being in effect a true mechanical Atkinson cycle having potentially the increased efficiency and pumping loss reductions associated with this cycle.

Interestingly, Gheorghiu [19] using computer simulation, compared a pseudo Atkinson cycle used in a contemporary hybrid car and a true Atkinson cycle which would be the case for the Lugo engine. “The potential of the Atkinson cycle realized through extended opening of the intake valve was investigated in a number of different variants. Analysis of the simulation results shows that the benefits [of the pseudo Atkinson] in the form of increased efficiency would be minimal in this kind of cycle”.

Gheorghiu [19] subsequently stated: "As an additional variant, the use of a new kind of crankshaft drive was presented which permits different size strokes for compression and expansion. In this case the improvements in the efficiency are clearly visible even without supercharging of the engine”. The theoretical crankshaft that Gheorghiu [19] proposes in his research is very similar to the one in the Lugo engine.






All of the VCR-related patents mentioned above [2,3,4,5,6,7,8,9,10] (except [2] which uses a different mechanism) use a fixed outer ring gear with the cam attached to a rotating pinion gear. They vary the compression ratio by means of a lever mechanism to rotate the ring gear, back and forth, through a small angle. Claims 11 and 22 of reference 16 show that Lugo can vary the compression ratio, if desired, by attaching a similar lever mechanism to the pinion gear






A prototype Lugo design using SI was retrofitted into a standard two liter production engine with a 12:1 CR as shown in figures 8 to 12.  The Intake camshaft lobes were modified to accommodate the reduced intake cycle of 160º and the intake manifold was consequently modified to handle an intake volume reduced from two liters to one liter.


Although only limited testing was possible, it allowed a preliminary demonstration of the main advantages of this design:

The engine ran smoothly from idle to 3,500 rpm (the maximum speed at which the prototype was run).

There was minimal ‘noise, vibration and harshness’ (NVH), and no detectable vibration. This was particularly pleasing as, in correspondence, engineering colleagues had suggested that NVH could be a problem due to the rotating crown cams. In this context it should be noted that no attempt was made to ‘counter balance’ any of the rotating parts. 

It appears that, unlike a conventional reciprocating engine, where unbalances tend to add up with each revolution of the crank, the Lugo engine crown cams are never in the same place during two revolutions of the crank, so there is a beneficial cancelling effect instead of a cumulative effect that would lead to vibration.




Design analysis and preliminary prototype test results suggest that the proposed Lugo engine development could offer significant advantages when configured solely as an Atkinson Cycle.

However, the principal attraction of the Lugo design is its potential ability to handle HCCI in a practical manner, providing an opportunity for much more efficient and flexible multi-fuel engines with reduced emissions. Such a result would hold great promise for the automotive industry and its environmental challenges.

The VCR feature of the Lugo engine although not appraised in the prototype tests remains as an added plus to the engine designer

The next step would be the construction and testing of a fully instrumented Lugo engine from scratch (rather than a retrofit) in order to properly test and confirm the advantages gleaned in the initial prototype.



US Patent Nº 367,496, GAS ENGINE, Atkinson, J. August 2, 1887


US Patent N° 1,553,009, ENGINE, E. Stuke Sept 8, 1925

US Patent N° 6,349,684, CRANK-CONNECTING ROD MECHANISM, De Gooijer, Lambertus Hendrik, Feb 26, 2002

US Patent N° 4,044,629, RECIPROCATING PISTON MACHINE, Clarke, John Michael Aug. 30, 1977

US Patent N° 4,966,043, CRANK DRIVE, Frey, Heinz, Oct. 30, 1990

US Patent N° 5,158,047, DELAYED DROP POWER STROKE INTERNAL COMBUSTION ENGINE, Schaal, Jack E, Schaal, Robert G, Oct. 27, 1992


US Patent N° 6,408,814, FOUR-CYCLE INTERNAL COMBUSTION ENGINE, Shighemori, Yoshiharu, Jun. 25, 2002

US Patent N° 7,293,542, MOTOR WITH ROTARY CONNECTING ROD BOLT, Ozdamar, Hasan Basri, Nov. 13, 2007

Roland Gravel, “New engine design adjusts compression to meet driving conditions”,

S. Onishi, S.H. Jo, K. Shoda, P.D. Jo, S. Kato, “Active Thermo-Atmosphere Combustion (ATAC)-A New Combustion Process for Internal Combustion Engines”, SAE 790501, 1979

Paul M. Najt, David E. Foster, "Compression-Ignited Homogeneous Charge Combustion," SAE paper No. 830264, 1983

Xingcai Lu*, Yong Qian, Zheng Yang, Dong Han, et al., “Experimental study on compound HCCI (homogenous charge compression ignition) combustion fueled with gasoline and diesel blends”, Key Lab. for Power Machinery and Engineering of M. O. E., Shanghai Jiao Tong University, 200240 Shanghai, PR China, 20 October 2013.

Epping, K., Aceves, S., Bechtold, R., and Dec, J., "The Potential of HCCI Combustion for High Efficiency and Low Emissions," SAE Technical Paper 2002-01-1923, 2002, doi:10.4271/2002-01-1923.


U.S. Patent Application Serial No. 13/109505, Filed: May 17, 2011, Entitled: S. Perez et al. "VARIABLE STROKE MECHANISM FOR INTERNAL COMBUSTION ENGINE"

PCT/IB2012/001882, S. Perez et al. Entitled: "VARIABLE STROKE MECHANISM FOR INTERNAL COMBUSTION ENGINE", March 13, 2014

Victor Gheorghiu, "Enhancement potential of the thermal conversion efficiency of ICE cycles especially for use in hybrid vehicles," Hamburg University of Applied Sciences, Germany,




Simon Perez
President and CEO
Msc Stanford University 1979

Henrique Perez
Chief Licensing Officer
B.Eng. Sheffield University 1966




















Bottom Dead Center

Compression ignition direct injection

Compression Ratio

Exhaust Gas Recirculation

Homogeneous Charge Compression Ignition

Internal combustion engine


Oxides of nitrogen

Noise Vibration and Harshness

Particulate matter

Reciprocating Piston Engine Impediment

Spark Ignition

Spark ignition direct injection

Top Dead Center

Variable Compression Ratio