Disclaimer: I am not receiving any remuneration for this article. Many of the statements and conclusions are based on my interpretation of current public data and are neither confirmed nor refuted by Mazda or any other institution.
Introduction
Mazda is well-known for taking risks when it comes to the internal combustion engine. Their implementation of the Wankel engine is a prime example of what can be achieved within an innovative environment. It comes as no surprise that in June 2019, Mazda announced the market introduction of a new type of engine, the Skyactiv-X. Uniquely it operates in SPCCI (SPark Controlled Compression Ignition) mode also known as SACI (Spark Assisted Compression Ignition). In this mode, the chemical kinetics of the air-fuel mixture are of far higher importance than in traditional spark-controlled gasoline engines. The result is the ability (and requirement) to run very lean, mitigate knock, and enable faster combustion. The exact operating region of the SPCCI mode is not publicly known, but from some published data (see Figure 1) it would appear to operate over much of the low-mid speed and low-high load range, for use over drive cycles [1].
Cost and Technology
Details of the engine were announced in a recent press release and are shown in Table 1 [2]. The largest unknown, at least for me, was the final compression ratio. At 16.3:1 it is significantly higher than the 2.5 L Toyota hybrid application at 14:1. A high compression ratio is required for kinetic-rate controlled combustion and it yields higher efficiency with the high expansion stroke. Engine-knock is often associated with high compression ratios, however the SPCCI operating mode (very lean) seems to avoid the knock problem. However, the engine does not operate in SPCCI under all conditions. At higher speeds and loads a traditional spark-ignited combustion process is used. It is possible fuel economy may suffer in this region as more spark delay is utilized to avoid knock at the high geometric compression ratio. This could potentially lead to large discrepancies between homologated fuel economy and real-world driving. The total power of 132 kW is enough to shift from 0-60 in around 7.0 seconds, ample for an efficiency-biased vehicle. The fuel-type recommendation is 95 RON. Of interest is use of the word ‘recommended,’ which suggests that Mazda will have provisions in place for those using lower quality fuel. It can be assumed that the efficiency penalty for running a lower octane fuel will be larger than for a traditional IC engine.
The pricing of the Mazda3 appears competitive with its efficiency and performance offerings. However, the price is around $4,500 more than a typically priced traditional gasoline engine (see Figure 2). The pricing structure shows that the vehicle is at a higher cost than the diesel (Skyactiv-D) offerings. However the Mazda3 Skyactiv-X beats the diesel’s 99g/km CO2 by 3 g/km, and with a faster 0-60 mph time the higher price may be justified [3].
Why is the price as high as it is? An analysis of engine pictures and press-statements gives an indication of the high-level of technology on the engine (Figure 3). We can see:
Diesel-like fuel injector lines suggesting fuel pressure is far higher than the 250-300 bar that most gasoline vehicles have. Rumors say it is upwards of 700 bar but I cannot confirm this.
The engine has electronic valve timing to allow precise control of residuals.
In-cylinder pressure sensing is interesting to see on the engine and is likely a high-cost item, but necessary for the SPCCI combustion mode.
Likely a high-power ECU will be required to cope with this requirement.
An EGR cooler.
A supercharger.
24 V B-ISG [4].
This rounds out the technology that can be seen. Additionally, running lean will require diesel-like aftertreatment of lean NOx trap (LNT) or selective catalytic reduction (SCR), or both, again adding a large cost to the vehicle. Clearly this is a high-content, high-cost engine which Mazda hopes will battle heavier-hybrids and full battery electric vehicles.
Mazda was awarded US patent 9,719,441 on Aug 1 2017 (Figure 4) [5]. The title of the patent is ‘Control Device for Compression Ignition-Type Engine’. The image of the combustion chamber suggests that this is an SPCCI-related patent rather than pure compression ignition, i.e. diesel. The following analysis assumes these features are present in the Skyactiv-X engine. The two most interesting parts for me are the valve-timing flexibility and the Ozone (O3) generator. Figure 5 shows the potential range of the intake and the exhaust camshaft. The exhaust appears to have a low lift operation well into the intake stroke. It can be assumed that this, combined with the supercharger, gives some additional control on trapped residual content, useful for controlling the SPCCI process. The intake cam has a high and a low-lift condition but without knowing abscissa values it is difficult to assume early or late intake valve strategies.
The Ozone generator can be seen in Figure 6 (item 76). The patent claims this helps to control oxygen content within the intake stream to permit further control over the SPCCI process where ambient conditions vary. A pure guess would be that an Ozone generator may offer some assistance in helping to meet N2O standards in the US, although that is not alluded to in the patent.
Efficiency and Performance
Comparing efficiency and performance to previous generations is also possible with the graph presented in Figure 7 [1]. Although the efficiency y-axis is not shown I have made some assumptions from historic data of the Skyactiv-G and MZR engines. I tentatively estimate the peak brake thermal efficiency (BTE) of this engine at around 40-42%. The data are shown at 2000 rpm--typically gasoline engines have peak BTE between 2500 and 3000 rpm, so it is possible the actual peak efficiency lies between 41-43%. I assume 42% going forward in this article. 42% is significantly (2% points) higher than the next best gasoline engine on the market, again the 2.5 L Toyota TNGA engine. Clearly, peak BTE is not a meaningful number over a drive-cycle and that’s where the engine really shines as we will see from the cycle efficiency later.
The efficiency number really starts to look impressive when compared against power density. The graph below (Figure 8) is often presented by Toyota [6]. On it I have placed the Skyactiv-X positioning with my 42% assumption. The Skyactiv-X has literally moved the bar for what is achievable. Consider that even with the excellent engine design from Toyota, power density would be reduced to 44 kW/l to achieve Mazda’s 42% or that efficiency would be sub 40% to achieve the 66 kW/l target. Moving those points up AND to the right is what is particularly impressive about this engine.
Having an efficient engine alone is pointless. Until it moves a vehicle it is just an expensive heater. I chose to compare the Mazda3 to two Toyota vehicles both hybrid and non-hybrid versions. [7]. I will discuss later that the Mazda3 will technically be a ‘hybrid’ as it comes with a P0 belt integrated starter generator (BISG), however the capability of the BISG on this application is unknown. The potential fuel economy reduction from running in hybrid mode must be considered for a fair assessment. I will assume the P0 gives 5% fuel economy benefit for the Mazda3. The vehicle compares very favorably to the Toyota non-hybrid variants, it produces around 20% less CO2 than the equivalent class vehicle (Corolla) and 40% less than the larger Camry (Table 2). However, compared to the hybrid variants the Mazda3 does not appear as competitive, this is likely due to the larger utilization of hybridization in the Toyota products. In this comparison I am most interested in the ‘non-hybrid’ results where it appears the Skyactiv-X engine operates very efficiently over drive-cycles to be significantly lower than the Toyota engine. The Toyota peak efficiency for the non-hybrid versions is 39% BTE which is around 7% lower than the Mazda’s assumed 42%. So, the other 13% benefit must come from other areas on the map. This leads me to believe a significant portion of the map is >37% BTE.
Markets
The engine will come to Europe initially in the CX-3, Summer 2019 (anytime now). It will then be released in Japan during Fall 2019. No release date for the US has been given yet. I am not sure if it will ever be released in the US owing to the strict NOx and N2O requirements set-out by SULEV emission standards. Achieving N2O standards is difficult with low temperature exhaust conditions as would be expected in this application. To be NOx compliant will likely mean a larger LNT or SCR system and more regeneration time which eats into the fuel economy benefit significantly. As the US typically does not pay for fuel efficiency it is hard to see the business argument of releasing the vehicle in the US.
Conclusion
Mazda, a firm noted for its out-of-the-box thinking, has made a special engine. It is far ahead of other technologies. However, that technology comes at a high price, literally. One wonders whether a more typical hybrid approach, like big-brother Toyota has been successful with, would be the better strategy when considering drive-cycle CO2. I applaud Mazda for their attitude towards keeping the internal combustion engine alive. I hope it stands as a testament to what engineers can do when they get a chance to throw out the rule-book.
References
Schultze,C. (2017) Skyactiv-X and innovative gasoline engine with compression ignition [PowerPoint presentation] Retrieved from: https://www.inspire.ethz.ch/site/assets/files/2035/6_c_schultze_mazda.pdf
Mazda (2019 June 5th) Revolutionary Mazda Skyactiv-X engine details confirmed as sales start [Press Release] Retrieved from: https://.www.mazda-press.com/eu/news/2019/revolutionary-mazda-skyactiv-x-engine-details-confirmed-as-sales-start/
http://www.cars-of-europe.com/mazda/mazda_3_kt108.shtml Accessed June 10th 2019
Malan,A (2019 June 12) Mazda opens orders for Mazda3 with fuel-saving Skyactiv-X tech https://europe.autonews.com/automakers/mazda-opens-orders-mazda3-fuel-saving-skyactiv-x-tech
US Patent 9719441, Control Device for Compression Ignition-Type Engine, Mazda.
Mori A et al, The New Toyota V6 3.5L Turbocharged Gasoline Engine, 26th Aachen Colloquium, 2017
Toyota Online Resource: https://www.tanskysawmilltoyota.com/compare-2019-toyota-camry-vs-2019-toyota-camry-hybrid-columbus-oh.html: Accessed June 18th 2019