1. Introduction
The pursuit of the potential benefits of CAI whilst simultaneously avoiding the accompanying challenges has led to the development of numerous CAI realisation strategies.
Two-stroke gasoline engine researchers were the first to study CAI
[1][2][3][4]. They were interested in it as a means to stabilise low load performance. The inherently high fraction of hot residual gases that are trapped after scavenging in two-stroke engines were exploited to initiate multi-site CAI. Thus, improving the quality of combustion and reducing HC emissions, which were exceptionally high for the port fuel-injected (PFI) or carburetted engines. Even though two-stroke CAI achieved some commercial success
[5], the cost of managing exhaust HC emissions via combustion and thermal efficiency improvements was too high compared to using valved, four-stroke engines that did not suffer from the gas-exchange problems inherent to the two-stroke architecture. Two-stroke engines have witnessed a resurgence in interest in recent times because of their potential to extend the high-load CAI operating limit
[6][7][8][9][10].
Diesel engine researchers investigated CAI as a means to reduce engine-out NO
x and soot emissions by breaking the so-called ‘NO
x-soot trade-off’
[11][12][13][14]. They hoped to avoid diffusion combustion while retaining the high efficiency of CI engines. Diesel CAI was an active research area at the turn of the century, but it has seen a downturn in the post-dieselgate era because of the general disfavour exhibited towards diesel engines by the automotive market. Some of the most promising gasoline CAI technologies have their roots in diesel CAI research.
Gasoline four-stroke engine researchers are interested in CAI as a means of achieving diesel-like efficiencies and lowering NO
x emissions, while retaining the low PM emissions of traditional SI engines
[8][15][16][17]. A secondary motivation is to stabilise engine combustion at low loads to mitigate cyclic variability by replacing deflagrative combustion with more robust multi-site AI. A tertiary motivation is the desire to reduce costs, relative to diesel engines, by using cheaper, low-pressure fuel injectors and cost-effective aftertreatment devices
[18].
A characterisation of various CAI strategies and a discussion on their relative strengths and weaknesses is presented next.
2. Strategy 1: Homogeneous Charge Compression Ignition (HCCI)
This is the original ‘best of both worlds’ CAI engine concept that is a true hybrid between a conventional DI CI diesel engine and a pre-mixed SI gasoline engine. In fact, most of the discussion presented so far for CAI is directly applicable to HCCI. The use of HCCI was avoided so as not to couple CAI to just one implementation mode.
In HCCI, a diluted, lean, or stoichiometric, fully premixed (homogeneous) fuel-air-residual gas mixture is made to combust via AI by making the cylinder’s thermodynamic conditions conducive for spontaneous, self-sustaining combustion. Since the composition of the mixture is homogenous, ignition begins at multiple sites distributed uniformly across the combustion chamber. The resulting energy release is volumetric and takes place at a near-constant volume around the top dead centre (TDC). The thoroughly mixed fuel and air, which is usually in excess of stoichiometric, prevent PM production, and the low combustion temperatures thwart thermal NOx generation.
2.1. Mixture Preparation
Four-stroke cycle-based gasoline HCCI engines achieve homogeneous mixing via PFI
[16][17][19] or DI during the intake stroke
[8][15]. Recent trends have leaned towards intake stroke DI because of improved mixing and better mixture preparation control.
Preparing a homogenous mixture is more difficult using diesel because of its poor vaporisation characteristics. Early diesel engine HCCI research explored homogeneous fuel-air mixture preparation via PFI to allow greater time for air-fuel mixing
[20][21]. Later research efforts have preferred DI during compression and expansion strokes because of improved fuel vaporisation and combustion control.
Two-stroke cycle HCCI engines have traditionally achieved homogeneity between fuel and fresh air from the direct metering of fuel into the air stream, which was then churned in the crankcase before being pushed into the combustion chamber. Newer two-stroke CAI engines use DI
[7][10].
2.2. HCCI Limits
The two most-significant hurdles that have limited the adoption of HCCI engines result from its most prized feature, its combustion mode. The two hurdles are: (i) a lack of ignition and combustion phasing control, and (ii) very rapid combustion at high loads. To overcome these hurdles, multiple solutions have been proposed that try to retain as much of the efficiency and emission benefits of the best-case scenario technology. These are the CI-suffixed CAI strategies and are discussed next. The use of the suffix CI (instead of AI) is likely an artefact of the historical efforts to deploy pre-mixed, auto-ignited combustion in CI (diesel) engines. These strategies drift from the HCCI vertex of the combustion mode triangle towards SI or CI as they attempt to address the shortcomings of HCCI by borrowing combustion management tactics from either/both of the conventional combustion modes. In one form or another, all of these strategies try to:
3. Strategy 2a: Premixed Charge Compression Ignition (PCCI)
Premixed charge compression ignition (PCCI)
[11][22] or premixed compression ignition (PCI)
[12][14], is a diesel engine strategy that came out of the challenges associated with creating homogenous fuel-air mixtures in diesel engines and the need to control and moderate HCCI combustion. In this approach, premixed rather than homogeneous cylinder gas mixtures are created by injecting fuel directly into the cylinder typically during the compression stroke at relatively advanced timings (e.g., 60°CA bTDC in
[11]). This provides more time for mixing relative to diesel combustion to avoid very rich pockets, while still having relatively rich strata available to initiate AI
[12][14]. The fuel-air stratification reduces the reliance on chemical kinetics for combustion initiation, and the existence of an air-fuel ratio distribution across the cylinder moderates the energy release rates. Additionally, the premixed nature of background cylinder contents reduces soot emissions; and if appropriate levels of dilution via EGR are used, NO
x emissions can also be kept low.
In the literature, diesel engines with early compression stroke injection have sometimes
[13][23] been referred to as HCCI engines, but here, they are referred to as premixed to be consistent with the mixture preparation process requirements for DI HCCI engines defined earlier, i.e., injection during the intake stroke. Moreover, as there are non-negligible heterogeneities in the combusting mixture’s composition even with the relatively advanced injection, PCCI is considered a more apt characterisation.
Fuel Injection
Most premixed CAI strategies
[13][23] (including gasoline strategies discussed under strategy 2b
[24][25]) employ multiple fuel injections as they allow for increased control over combustion timing and phasing. With multiple injections, the initial injections help prepare a lean, premixed background charge that is ignited by a relatively rich, combustion-triggering injection pulse close to TDC when cylinder gases are compressed and have higher temperatures. By splitting the fuel admission, energy release is spread across a wider crank angle window to avoid very rapid PRR, thus allowing more fuel to be added at higher loads without breaching PRR thresholds. The timing of the initial injection event is important. Early injection during the compression stroke when the cylinder gas density is relatively low results in higher jet penetration, which increases the likelihood of fuel adhering to combustion chamber walls
[11]. This, in turn, can cause an increase in PM emissions. Therefore, designing appropriate injectors
[12] and combustion chambers
[14] to reduce wall wetting and/or using suitable split-injection strategies to deliver fuel in multiple, smaller jet parcels are important strategies.
The timing and duration of the AI triggering injection pulse is also important, as it determines the gap or overlap between the fuel injection and heat release events, i.e., the ignition dwell. It is preferrable to have ignition dwell, which is often taken to be an index of the extent of diffusive combustion and PM emissions
[18][23], be positive. With split injection strategies, the final pulse can be shortened by metering most of the required fuel through earlier injections to maintain a positive dwell. High injection pressures can also help maintain the desired ignition dwell by injecting the required amount of fuel in a shorter period
[18]. There can be accompanying fuel mixing benefits from high-pressure injection. Very high pressures can, however, cause excessive mixing, whereby the local equivalence ratios can become too lean to support combustion, and the long penetration lengths of high-pressure jets can cause an increase in PM emissions through increased wall impingement. Injection timings can have different effects on emissions depending on the specific engine combustion chamber design and injection event details. Some general trends summarised from Bobi et al.
[26] are as follows: advancing injection timing causes an increase in HC CO emissions because of increased wall wetting and crevice entrapment, and it lowers NO
x and PM emissions because of reduced combustion temperatures.
High levels of EGR have been used in PCCI engines to reduce mixture reactivity and delay the start of heat release after injection
[27][28]. High EGR diesel combustion can be considered analogous to using a low reactivity fuel, such as gasoline, in diesel engines, as both EGR and gasoline reduce the mixture’s auto-ignitability. This is the basis of strategy 2b.
Another way to moderate PRR is to retard combustion phasing, but in HCCI combustion, retarded combustion has been found to be very unstable, both in diesel
[29] and gasoline engines
[19]. A late injection event in multistage injection PCCI can help stabilise retarded combustion by providing relatively rich pockets to anchor combustion across consecutive cycles. PCCI has sometimes been realised by very late (post-TDC) injection to have long ignition dwell and to promote thorough fuel-air mixing
[27][30].
4. Strategy 2b: Partially Premixed Charge Compression Ignition (PPCCI)
PCCI attempted to temper diesel HCCI to make it more manageable and controllable. Diesel, however, is an inherently poor fuel choice for HCCI combustion. Its high autoignition propensity (high CN) makes it susceptible to premature AI during fuel-air mixing processes at typical diesel engine compression ratios. According to Kalghatgi et al.
[18], a compression ratio of 10:1 would be needed to allow injected diesel sufficient time to mix with air to form a well-mixed mixture. Conducting this so would, however, remove one of the biggest thermodynamic advantages that diesel engines possess and thus lower their efficiency. Hence, a low CN fuel, such as gasoline, can be a more-appropriate choice for moderating HCCI. This is the basis of the PCCI concept adaptation for gasoline engines, which is referred to as partially premixed auto-ignition (PPAI)
[23] or partially premixed compression ignition (PPCI)
[24].
PPCCI can be realised in engines with compression ratios typical of diesel engines if thermal assistance is available (e.g., as intake air heating or charge heating via iEGR) to promote the AI of gasoline. Without any thermal AI assistance, gasoline fuels would require compression ratios in the range of 18–20:1 for AI. PPCCI engine compression ratios range from 16 to 18:1
[24][31][32][33]. Compared to diesel engines, PPCCI engines require relatively low fuel injection pressures because of gasoline’s volatile nature, which reduces the need for atomisation to vaporise fuel, and the relatively greater time available for mixing.
In PPCCI, similar to PCCI, heterogeneity is purposefully introduced in the cylinder mixture to create a non-uniform equivalence ratio distribution to moderate and stabilise combustion. From the surveyed literature, this is always performed using multiple direct injections
[23][31][32][33][34][35][36]. The first fuel injection typically takes place during the intake or early compression stroke (e.g., 150–170°CA bTDC
[23][33]) but can take place later too (e.g., 26–31°CA bTDC
[25]). According to Kalghatgi et al.
[18], for high-ON fuels, such as gasoline, injection at 60°CA bTDC is sufficient to form homogenous mixtures for combustion. The first PPCCI injection is followed by additional injections (typically 1–3), with the last one phased around TDC to serve as an AI trigger (e.g., 16–0°CA bTDC
[33]).
The intermediate injection(s), if present, create(s) a lean stratified charge, and the last injection generates a rich cloud that ignites relatively easily (shorter ignition delay). The triggering fuel injection yields a relatively rich mixture, e.g., the third injection in Liu et al.
[31] prepared a fuel-air equivalence ratio 0.49 mixture, compared to 0.24 and 0.2 for the second and first injections, respectively. The proportion of fuel mass injected during the pilot and main injection events also affects engine efficiency and emissions and combustion noise
[33]. Thus, injection parameters have to be selected to realise appropriate levels of fuel stratification in a globally lean mixture to optimally control ignition and combustion progression. Dempsey et al.
[35] compared the relative performance of a PPCCI engine across a range of stratification levels.
Recently, a new variant of PPCCI that incorporates diffusive combustion in addition to AI has been demonstrated by
Sellnau and co-workers to extend the high load limit in a heavy duty engine to as high as 23 bar BMEP
[37][38][39]. The strategy, referred to as ‘PPCI-diffusion’ has a late compression pilot injection (30–40°CA bTDC), which triggers an AI stage, followed by a second injection close to TDC, which initiates a diffusive combustion stage. A similar two-stage (AI + diffusion) gasoline combustion strategy was used by Hanson et al.
[7] in a two-stroke opposed-piston PPCCI engine at high load points. High diesel-like injectors with injection pressures ranging between 1000 and 2000 bar were used in both the PPCCI-diffusion combustion engines.
PPCCI Limits
While PPCCI enables the extension of
high load limits, there are still limits to this extension, as at very high loads, PRR becomes prohibitively high. Therefore, EGR is also used to moderate energy release rates; to counter the effects of excessive dilution, PPCCI engines are generally supercharged to further extend the high load limit
[23][32]. Recent PPCCI work has also used ‘e-boost’ for operating the engine at high loads
[31].
Since PPCCI engines do not have a spark plug, they cannot switch to conventional SI operation at high loads, which would be analogous to PCCI engines reverting to conventional CI at very high loads, where PRR could not be moderated using EGR, boost, and multiple injections
[13][14][27][28][30]. This is one of the motivations behind the development of spark-assisted compression ignition engines (strategies 3 and 4). PPCCI engines can potentially switch to a different ‘conventional’ combustion strategies, namely diffusive (CI) combustion at the highest loads
[7][37].
In PPCCI engines, low load operation can be a challenge because of gasoline’s poor AI characteristics and the relatively low temperatures at low loads. A few approaches that can enable stable low-load operation in PPCCI engines are:
-
Using variable valve actuation techniques, such as exhaust rebreathing
[24] or negative valve overlap
[34], to raise cylinder temperatures by trapping or recycling hot combustion products (iEGR) to promote AI.
-
Using a spark or a glow plug to assist with the combustion of the stratified mixture created by a late injection event
[32]. This is different from spark-assisted compression ignition because ignition phasing is controlled by the late injection event and not the spark discharge.
-
Avoiding low load operation by deactivating cylinders in multi-cylinder engines, e.g., in Cracknell et al.
[32] one of the four cylinders was deactivated at low loads, and up to a 33% improvement in fuel consumption was reported.
The benefit of the latter two approaches is that they do not require expensive variable valve actuation systems and can be implemented using standard engine devices.
5. Strategy 3: Spark Assisted Compression Ignition (SACI)
Another CAI strategy that was developed to address the shortcomings of HCCI combustion is spark-assisted compression ignition (SACI)
[15][17], which is also referred to as spark-controlled compression ignition (SPCCI)
[40][41] or spark-ignited compression ignition
[19]. In it, combustion is triggered in a well-mixed CAI (i.e., HCCI) engine via spark discharge, and the subsequent energy release is moderated by retarding combustion phasing and employing EGR and supercharging as needed.
The sequencing of combustion in SACI engines is as follows: a near-homogeneous mixture is prepared by early (generally during the intake stroke) direct fuel injection. A spark discharge event around firing TDC triggers deflagrative combustion. The advancing flame causes the temperature of the cylinder charge to rise through compression heating, which lowers the mixture’s ignition delay and eventually triggers AI. Thus, a two-stage combustion process is realised, comprising an initial slow deflagrative stage, in which some of the fuel’s energy is released, and a second AI stage that rapidly burns the remaining fuel present as a lean, homogenous fuel-air-residual gas mixture. The first stage is similar to normal combustion progression in GDI SI engines, whereas the second stage is similar to abnormal SI combustion caused by end-gas AI (i.e., knocking). Instead of being avoided, end-gas AI is purposefully initiated and managed by creating favourable conditions for its onset, e.g., higher compression ratios and warmer in-cylinder conditions.
The phasing of SACI combustion is controlled by: (i) spark timing, (ii) the speed of flame-propagating combustion and the fraction of energy released through it, and (iii) the ensuing AI, the nature of which is determined by the composition and thermodynamic state of the post-deflagration mixture. Of these determinants, the principal is the spark timing. Having the correct mixture state, composition, and in-cylinder flows at the time of spark firing is thus critically important. Instabilities in initial flame propagation and early kernel growth at the dilute and lean conditions typical of HCCI combustion are major sources of cyclic variability in SACI combustion
[17][42]. This is particularly problematic at low loads where the mixture temperatures are low and cause the further slowing down of flame initiation processes
[17]. Olesky et al.
[15] found that, for constant combustion phasing, AI stage reactions started at around 765 °C, irrespective of the spark timing.
The nature (speed and proportion) of the deflagration combustion stage can be tailored by controlling the temperature and composition of the cylinder mixture and, through that, the onset of spark-assisted AI. The flame propagating stage can be made longer at higher loads to avoid excessive PRR, e.g., by increasing external EGR (eEGR), decreasing iEGR, or decreasing intake air temperature or pressure. In Olesky et al.
[15], 18 to 34% of the total energy release was made to take place via SI by adjusting the ratio of iEGR to eEGR. In Xie et al.
[17], iEGR was reduced by decreasing positive valve overlap, which resulted in heat release becoming similar to that of SI combustion.
Since SACI engines are equipped with a spark plug, they can transition to conventional SI operation in regions where CAI is not feasible by making appropriate adjustments in the mixture preparation and gas exchange processes, e.g., switching to an overexpanded (Miller-type) cycle, mixture enleanment, and/or increasing EGR to avoid SI knock
[43]. Operating range coverage examples from SACI engine studies are shown in Figure 8.
6. Strategy 4: SACI-PPCCI Hybrid
It is categorised separately because it utilises moderating influences from both of its constituent CAI strategies, stratification via multiple DI events from PPCCI and deflagrative combustion from SACI. In the literature, however, it is more commonly referred to as SACI implementation
[19][40][44]. Strategies such as SACI-PPCCI hybrid are reminders that various means of characterising CAI strategies are merely tools to facilitate structured discussions on the topic and are not constraints to CAI implementation.
In the SACI-PPCCI hybrid strategy, a late second injection event takes place shortly before (or at the same time as) spark firing in an SACI engine
[42]. The additional injection introduces localised stratification around the spark plug with a homogenous background charge. This is conducted to have locally rich strata to make SI more stable and to accelerate flame propagation
[19][45]. The stabilisation provided by the second injection makes operation at more retarded spark timings possible, where typically lean and diluted HCCI mixtures experience very unstable combustion
[19][29]. Retarding combustion phasing by decreasing iEGR without additional support from fuel stratification reduced combustion stability in Olesky et al.
[15]. SACI-PPCCI hybrid engines can have more than two injections, e.g., Hu et al.
[45] employed 1–3 early injections around 300°CA bTDC to form the background well-mixed mixture.
Flame imaging experiments by Hu et al.
[45] found that the pre-spark injected fuel in a ‘partially fuel stratified’ engine acted as a locally rich diffusively burning ‘super igniter’. The injector was located such that the fuel jets straddled the spark plug electrode. The resulting energy release was three orders of magnitude higher than that from a standard spark discharge. The following deflagrative stage burned a well-mixed lean mixture, and the final AI stage consumed the remaining very lean end-gas. The resulting HRR profile for such SACI-PPCCI combustion is a combination of PPCCI and SACI profiles and has three stages: a diffusive PPCCI stage, a deflagrative SACI stage that has lower HRR than the preceding diffusive stage, and a stronger late AI stage. The nature of the first combustion stage can vary depending on the spatial and temporal distribution of the stratified fuel mixture around the spark plug. Flame imaging results from Reuss et al.
[42], in which fuel injection jets were targeted just below the spark plug gap and stratification was achieved using spray-guidance (instead of wall or flow guidance
[46]), did not reveal a diffusive combustion stage.
SACI-PPCCI provides flexibility in tailoring energy release profiles by controlling fuel injection and spark discharge events that determine the fraction of each of the three combustion stages. CA50 was found to correlate strongly and linearly with spark timing, demonstrating that spark timing is a strong overall combustion phase controller
[45].
As discussed for PCCI and PPCCI, late fuel injections can cause diffusion combustion, which can increase NO
x and PM emissions. Higher than expected, yet still low, NO
x emissions observed in Yoshizawa et al.
[19] were attributed to the combustion of the relatively rich stratified mixture around the spark plug.
Operating Strategies
Because of the flexibility in tailoring combustion strategies, the engine operation of SACI-PPCCI can be switched between SI, SACI, SACI-PPCCI, HCCI, and PPCCI as needed to cover the desired operating region. Moderated HCCI strategies (SACI, PPCCI, or SACI-PPCCI) can be used at high loads to manage PRR, and HCCI can be used at low loads to benefit from its thermal and combustion efficiency advantages. If CAI is not possible at low loads, conventional SI operation can be used, and if PRR is excessive at very high loads, CAI combustion can be switched to SI with accompanying adjustments in mixture preparation and gas-exchange settings. An example of a commercial SACI-PPCCI engine is presented next.
Mazda’s SKYACTIV X
[40] engine and the e-SKYACTIV X
[41] variant for mild hybrid applications are the first class of four-stroke gasoline CAI engines to reach the market. They are GDI SI engines with high compression ratios (16:1 and 15:1, respectively) and use a split fuel-injection strategy to realise SACI-PPCCI CAI. The pilot injection takes place during the intake stroke, and the main injection occurs during the compressions stroke. The fuel is injected at the moderately high pressure of 500 bar, and the engines run very lean with an air-fuel ratio of 40:1 in CAI mode
[44]. The engines also use turbulence in the form of strong swirl and tumble flows to help vaporise the injected fuel and to optimally direct it around the spark plug. A clutched supercharger is used to boost the engine as needed and is disengaged when natural aspiration is desired. A variable valve timing system creates positive valve overlap to help scavenge hot exhaust gases or operate the engine on an over-expanded cycle by early intake valve closing as needed. The SKYACTIV X engine covers most (80%) of its operating range under CAI operation and switches to conventional stoichiometric SI operation at very high loads. This is probably because the PRR moderation provided by spark assist is not sufficient at the highest load points. The engines operate in SI mode during cold start and warm up periods as well, likely due to cylinder contents being too cold to achieve stable AI. A resulting fuel economy improvement of up to 30% has been reported. The rated torque and power for Mazda e-SKYACTIV X translate to a BMEP of 13.7 bar at 6000 RPM and 15 bar at 4000 RPM
[41].
BMEP was converted to net IMEP by assuming 90% mechanical efficiency.
-
SACI alone cannot adequately extend the CAI operating envelope. Assistance is needed from PPCCI to provide stable, lean combustion across a wide operating range.
-
SACI-PPCCI hybrid engines allow for using relatively high compression ratios because of the availability of two ignition control knobs (spark discharge and fuel stratification) that can provide effective PRR moderation.
-
HCCI can serve as a stable low- to medium-load combustion mode, and operation can be switched to SI operation at very high loads and cold engine conditions.
-
Adjusting iEGR to the eEGR ratio using various valve overlap strategies is an effective way of stabilising SACI operation and extending its load limits.
-
Greater in-cylinder flow control and turbulence, relative to non-SI CAI engines, might be needed to stabilise and accelerate combustion during the deflagrative stage.
7. Strategy 5: Gasoline Compression Ignition (GCI) [Redundant]
In the literature, the term gasoline compression ignition (GCI)
[7][18][19][31][35] or gasoline direct injected compression ignition (GDCI)
[24][32] is used as well to refer to gasoline CAI. It is more commonly used to refer to gasoline PPCCI engines
[22][24][31][32][35], but it has been used for gasoline SACI engines as well
[19]. A reasonable case for the use of the term for both the gasoline CAI strategies can be made, but its use in the PPCCI context seems more appropriate, as the technology is a direct offshoot of diesel engines and is often implemented directly in diesel engines. Additionally, there is no spark plug in the cylinder, and the prime initiator of AI is compression. For this reason, PPCCI engines typically have higher compression ratios than SACI engines. PPCCI-SACI hybrid engines have compression ratios between the two (around 15:1).
8. Strategy 6: Reactivity Controlled Compression Ignition (RCCI)
Reactivity controlled compression ignition (RCCI) combustion is a CAI strategy in which two fuels—a high-reactivity (high-CN) and a low-reactivity (low-CN) fuel—are used
[47][48]. Generally, the high reactivity fuel is diesel, and the low reactivity fuel is gasoline. Appropriate amounts of each are metered to tailor the reactivity of the combusting mixture to obtain desired energy release profiles. Natural gas can also be used as a low-reactivity fuel
[49]. Because the scope of the present work is focused on single gasoline fuel-based systems, RCCI is discussed very briefly below.
In RCCI, typically, a homogenous mixture is formed in the cylinder by the port fuel injection of gasoline. The mixture is then compressed, and because of its low reactivity, it does not auto-ignite during compression. AI is triggered around TDC in a conventional CI combustion-like fashion by the injection of diesel into the hot compressed mixture of gasoline and air. The diesel combustion triggers the AI of the homogenised gasoline mixture that combusts rapidly in a premixed combustion event. The amount of low- and high-CN fuels admitted, along with the high CN fuel’s injection timing, can be varied for various speed and load conditions to achieve stable combustion with low NO
x and soot emissions. The challenge with this approach is that it requires two fuels, which increases the system cost and level of complexity. Octane-on-demand technologies
[22] can potentially remove the need for carrying two separate fuels.
In a recently proposed RCCI variant ‘intelligent charge compression ignition’, the low-CN fuel is directly injected into the cylinder instead of being injected into the intake port. This offers better control over the low-CN–high-CN fuel stratification
[50]. Another RCCI variant being developed, especially for electrified powertrains, is the ‘dual-mode dual-fuel’ concept, in which the relative proportion of the two fuels and diffusive and premixed combustion stages are adjusted to cover a wide operating map
[51].